I got on the phone with Dirk Starksen at Advanced Clutch Technology. He has been in this business maybe even longer than I have, so I trust his judgment. He suggested I try the company's heavy duty clutch and flywheel combo. Soon enough we had the car on the lift and the transmission out, and in went the ACT parts.
While we were under there, we replaced the leaking factory cam plate with the lovely Raceseng billet replacement cam plate. That got rid of that pesky oil-drip burning smell we were getting. (If you are not familiar with this issue, the problem is a seam on the block that is improperly sealed from the factory, and oil drips out of it onto the exhaust manifold.) The Raceseng piece is so nice looking, it's a shame nobody but my mechanics will ever see the thing.
The new clutch solved my slipping issues. The lighter flywheel made the engine feel zippy and willing to rev. Since the new flywheel is a single-mass part, there is a good bit more drivetrain noise under certain situations, like in-gear coasting at high RPMs. But it's not like this car is a quiet luxury car in any case, so I don't mind.
Getting the car back to full strength happened just in time, as the snow came and we swapped on the Blizzak snow tires (on Subaru Legacy GT wheels). The car worked great through the cold weather, and the supercharged power just gets better and better as the thermometer drops.