Not jabbing at you. People just don't pay close enough attention to tire weight.
I run 30 carnivores at 33lbs a piece. When it comes time for a new set...I think I'll go back to a true 28" tire. The 1" of clearance gained isn't worth it to me for the extra ware it puts on the machine.
With 32's, you've geared up about 20%. And those STI weight 47lbs. Damn heavy by anyone's standards. Where as the carnivores weigh 37.
So you've geared up and added a ton of rotating mass and unsprung weight. It's tough on a arm bushings, wheel bearings and your clutching.
We've all been guilty of putting big tires on our trucks and losing a little pep and watched Gas milage take a hit. That's on a 250+ hp truck. An 80hp belt driven 2 cylinder really takes a big hit.
That being said. There's nothing magical about the dura clutch. They wear out and need service also. They were originally designed for running tracks, have excellent engine breaking and smooth take off. Almost like adding hydrostatic drive to your machine. What your currently experiencing now is lack of belt grip near full shift. And most likely happening in the secondary clutch due to the heavy tires.
Gillomens torque monster weights and mudder secondary spring will adress the belt slip. And what I'd go with. Especially since your dealership is standing behind it. It's easily user adjustable should you ever decide to add weight, different tires etcetera. And parts are readily available for your stock clutch should you ever need bushings, rollers,bearings.