I'm running 175/60R15 Kumhos up front with 14-16psi in them and have no issues without any shimming or adjustments of the fenders; the only thing I've done is to swap the hex head screws on the inside of the wheel arch/fenders out in favour of lower pan head screws because the hex heads protruded out into the wheel space about 10mm & the pan heads sit pretty flush, not leaving anything for the tires to scrape on..
I have heard/seen that some people running this sized tire at different pressures find that the tire flexes/distorts enough during hard cornering for the inside rear edge of the tire to scrape on the lower inside rear panel of the fender. I believe that this is happening because they are running pressures high enough that the tire cannot flex appropriately or quick enough during cornering so that the distortion caused by the cornering forces keep the tire distorted for longer than it would if the tire was running at slightly lower pressures. Certainly all those I've seen with this issue have been running pressures of 18-20 psi ir higher (because they felt that gave them 'more direct' steering, despite the clearly lower traction & performance evidenced on back to back slalom runs at different pressures) and the scraping stopped immediately they dropped 4 or so psi out of their tires, bringing their pressures down below 18psi. The front end of our Spyders really don't carry a great load, so they don't need a high pressure to help the tire support that load, and running pressures too high reduces the contact patch size & also forces the edges of the contact patch to lift during hard cornering, reducing it even further & holding that distorted shape for longer as the carcass of the tire rotates!! Slightly lower pressures help optimise the size of the contact patch & allow enough tire carcass flex to keep the tread contact patch firmly on the road surface instead of peeling an edge or side up & off the road, & releasing any carcass distortion almost immediately the tread leaves the road surface!!
However, as those of us who have studied tires, pressures, & their use on vehicles are well aware, tire pressures are not a simple cut & dried one size fits all thing! Like the rest of us who have studied this, I am very aware that while running 'stronger' car tires on a Spyder rather than the lightly constructed Kendas definitely needs lower pressures, it isn't all that hard to go
too low & drop your pressures below the optimum for the way YOU ryde & the tires YOU are running on your Spyder (or car, or whatever) so much so that while you will get better traction from the lower pressure, that traction comes at the cost of somewhat increased rolling resistance, sapping power & needlessly wasting fuel, as well as making the handling feel a little less than ideal cos it's so soft & spongy. Anything less than double digit pressure on a Spyder could be an issue this way! Still, the optimum tire pressure for
your tires is a very personal thing, very dependant upon the way you ride & the tires you run, so what works for me may not be exactly right for you; but you can & shouldn't be scared of adjusting your pressures up & down
a little from that optimum to work out what
feels best for you & works best for your tires & the way you want to ryde.
The 4psi rule can help get the optimum pressure for your tires; it aims for a 4psi increase in pressure after an hour's riding - more than a 4psi increase means your cold start pressure was too LOW, while less than a 4psi increase means your cold start pressure was too HIGH... OR you could use a temp sensor to check the tread temp at 3-5/7 or so places across the tread face & then check them again 30-60 mins later - an even temp increase across the tread face is what you want to see, while a hot strip on one side or the other of the tread face generally reveals an alignment or a loading problem; hot edge strips down each side of the tread indicates underinflation; & a hot strip in the centre of the tread indicates overinflation. These tools can help you arrive at the optimum pressure for your tires with regard to the way you ride, the load on your Spyder, the tires you have fitted, and the road surfaces you are ryding on, etc, resulting in you getting the best combination of ride, handling, traction, fuel economy, & tire life you can from those tires, and knowing that, you can generally make an informed decision on how that
FEELS to YOU, or whether it will provide what YOU want from your tires, & adjust the optimum pressures up or down a little accordingly.
Dropping your pressures a little
below the ideal can soften the ride at touch, increase puncture resistance, & improve traction on marginal surfaces like wet or icy roads, but it will come at a cost in terms of greater rolling resistance, higher fuel use, & more tire wear on the edges of the tire, & if you go too low it can cause adverse handling and may also cause overheating & risk rolling the bead off the rim. Similarly, increasing your tire pressures a little
above the ideal can improve the 'directness' of your steering (up to a limit, above which it just increases the potential for traction loss &/or rapid & unpredictable transition from understeer to uncontrollable oversteer or even causing the vehicle to roll!) but it comes at the cost of less traction & greater tire wear, increased risk of puncture from road debris or hazards, and a harsher ride, & if you go to far you can seriously compromise traction and handling as well as making the vehicle unstable at speed (or any speed!!)
Hope that lot hasn't added too much to your ponderings, but with any luck, it may have helped others too!
Ps: all other things being equal & with correct alignment, running slightly wider tires will generally increase the potential tire life, stability, & traction on dry surfaces that you can expect, as well as
usually improving ride & handling. Running 'stronger' & slightly wider tires at the right pressures does all that too, but
definitely will improve the responsiveness & handling of the vehicle as well as improving both wet & dry traction if you've got your pressures right, but it's
essential that you get the pressures close to right, if they are too high the tire won't be able to clear water off the road surface or be at the correct temps to grip the surface properly... Lots of advantages to going up a size in width, IF you get your tire pressures right! :thumbup:
But at the most basic, it is extremely easy - you check & set your pressures cold, you ride, then you check again to confirm that you set your tire pressures correctly; and you do that from a cold start often enough for you to be confident that your tires pressures are correct for you & your riding.... some might take 10 set/check cycles, some might take 30, some might even take more... most take somewhere between 10 & 20 before they are reasonable confident! 
hyea: Besides, a lifetime of service, studying, exploring, & gathering experiences doesn't need to end with retirement just because my body can't hack the pace any more; passing on whatever I've learnt helps me keep 'active'. 
