Immediate difference.
This is all subjective based on the car tire you choose... but with the right car tires I've:
Had way better dry traction front and back
Had way better wet traction front and back
Had way better standing water performance front and back
The Kumho that I tried on the rear of my GS lasted 11k miles - about 1k more than the OEM Kendas. The Kuhmo was a fine touring tire, but overall it's dry and wet performance was pretty bad. Only benefit of the Kumho for me was even tread wear across the entire tire width.
On the GS fronts the Bridgestone Potenza RE-92's were absolutely awesome.
On the GS the best rear I had was the Toyo Proxes 4 (no longer made). That tire was super sticky in the dry and it's wet and rain performance was incredible. Between a nail in one Toyo and selling the GS I never figured out what miles I could get to with the Toyo (on the GS)
On the F3-S I switched to the Federal Evos for the fronts. Did it early. Bit stickier than stock and better in the rain. I switched out the Kenda fronts at 4500 miles and to be honest I would have been ok keeping the OEM fronts, but decided to switch early.
On the F3-S rear I have once again a Toyo Proxes 4. My OEM Kenda was ready for replacement near 4500 miles. After 5500 on this current Toyo it looks like I might make 8k miles before needing a new rear. So I'm not getting great mileage out of the rears on the F3. I chalk that up to riding style. My Spyder doesn't do low heat around town puttering. When I ride I'm out for 8+ hours a day of riding twisties with hard braking and acceleration. No burnouts other than some wheel slip now and then.
Best advice? Any car tire is going to perform better than the Kendas and wear evenly. You may or may not get more miles out of the rear specifically. The Kumho is fine for many, but I suggest spending the extra $ on something better like the Yokohama s drive 205/50R15. It may even be in the same price range but people seem far happier with the Yokohama than the Kumho.
As for air pressure I'll only say you will find lots of opinions and a lot of apparent tire experts here. Yes the Spyder is lighter than a car and thus the tires aren't even pushing their load ratings. I've run 20-22 PSI on the fronts for thousands of miles with no issues and anywhere from 28-30 on the rear with no issues. You go too low of a PSI and the tire may create too much heat via sidewall flex and may break a bead in an abrupt swerve. Some here suggest some very low PSI in the tires.... "Tread" carefully... Get it?
I buy mine for performance and not price and not the miles I get out of them. True, I wish the tires would last longer on my F3-S but that is probably my fault more than anything else. Just remember that at some point after your 2nd or 3rd rear tire you should have your rear bearings inspected and potentially replaced.
I will be mounting my last Toyo Proxes 4 this summer at some point... I really wish they still made that tire.