The OE Muffler on those early Spyders had a Catalytic Converter cell included in the front/engine end of the muffler, and Cat Converters
NEED fairly high temps (650°C plus IIRC?!?) to properly activate the catalytic material in order for them to work properly. The HMF muffler almost certainly did
NOT have a Cat Converter included, which is why they are so much lighter and generally run at significantly lower temps! And, since the HMF didn't have the massive choke point in it that those Cat Converters are, it probably would've caused the engine to run a tad oddly without the juicebox, so adding that would've provided access to more power and reduced the banging & popping on decel as well as minimising the risk of engine damage from running too lean at anything less than WOT... and probably even if paradoxically it might have improved the low/mid range fuel economy too - in a nutshell, better power in the revs you use most means less wringing of the throttle & therefore returns better fuel economy despite the (marginally?) increased fuel delivery! :shocked:
All of which means that now you've re-fitted the heavy OE Muffler with it's integrated Cat Converter, it adds up to those high temps you're experiencing until you get up into higher throttle settings where the
air flow in is better - the OE Muffler with it's Cat Converter is now almost certainly generating a
LOT of that extra heat, especially at low revs, by choking the exhaust off (as it's designed to in order to generate the heat it needs to work) but that's then made worse due to the power loss caused by the more restrictive exhaust
and the extra fuel (now wasted & simply costing you fuel economy) that the juicebox is pumping into the engine with little effect, especially at low to mid-range revs where it's added power/fuel would've been most beneficial
IF it could be pushed out the exhaust, even with the standard ECU settings.......
So if you
really want to reduce the heat & get some of that fuel economy back, I'd suggest the
easiest/cheapest way to achieve that would be to keep the juicebox, toss the OE Muffler & it's integral cat converter (again), and re-fit the HMF muffler only with a baffle or two installed to reduce the sound output, or a quieter a/mkt free flowing alternative.

hyea: That'll also have the benefit of giving you back some of the low & mid range power you may not yet even have realised you lost by re-fitting the OEM Muffler & it's heat generating choke point of a cat converter & which then would be compounding by removing the juicebox & de-tuning the engine completely back to its std settings to suit! :lecturef_smilie:
It sounds like someone did a really good job with the HMF & juicebox re tuning it all properly for low-mid range power & fuel economy, only
they didn't mind the noise.... (probably deaf already?!

) so why waste all that good work & effort and return your Spyder to the de-tuned pre-mod mediocrity it left the factory with - just fix the noise from the HMF with a baffle or two; maybe even swap to a quieter a/mkt muffler if you must; but certainly try to keep the tuning, the low/mid range power, & the fuel economy improvements your machine already had that come with the juicebox & a free flowing exhaust without a Cat converter,
AND at the same time, avoid the excessive heat & added weight that comes with the OE Muffler & its integrated cat converter! :yes:
Just sayin', & playing the Devil's Advocate here! :2thumbs: