Back in the early 90's I had a buddy who was a proud owner of a Ford Mustang LX 5.0. I can confirm what is said in this tread is true. The stock clutch was fine as far as shutter until the day it was replaced with a centerforce dual-friction, and then the shutter began. In the beginning, we weren't sure what actually caused it, but we were told from other racers that heavy pressure plates with composite materials or potentially a misalignment of the clutch against the flywheel. This can be confirmed by removing the tranny (which we did) and inspecting the flywheel for hot spots. You know what, there wasn't any. A whole lot of speculation at first of a rear main seal leak, etc. When we originally replaced the clutch, I also replaced the rear main seal, throw out bearing, input shaft (Ford motorsport version), flywheel (aluminum and machined), clutch fork and Centerforce clutch (pretty much everything). It was a $600 excursion. After remachining the flywheel (for good measure) and checking for warp we put it back together. Same old shutter and we accepted this as normal. The car ran 11.40's with the ATI procharger and pulled the front tires 2 feet at launch. No shutter on those hard launchs
. But it would shutter ALL the time on the street in first gear under easy driving.
This being said, I believe that Subaru is putting in some heavy clutches into the WRXs. I just got my WRX today, but I assume it is a hydrolic clutch, which will make it hard for you to REALLY know how heavy the pressure plate really is. In my buddy's car (manual clutch) you had to have a leg like a body builder to push that thing in.