have you ever heard of a "zero clearance" mod to the compressor housing?
There's a shop up here called Ultimate Motorwerks that does this for the Porsche 993-996TT crowd. I'm not sure if they're still in business (we used to do wastegate springs for him but haven't heard from him in 4+ years). The idea was to coat the inside of the compressor housing with an epoxy which would allow the compressor wheel to run in contact with it to more-or-less self clearance the wheel to the housing and be able to run at the absolute bare miniumum of clearance. Claimed efficiency increases were 10 to 20%. Cost was like $450 per turbo.