John,

I assume that the drivetrain in the C70 is substantially the same as what's in my 850. I upgraded my 850's engine with a larger turbo, downpipe, exhaust, custom ecm and injectors right around 100,000 miles. I would estimate my hp to be somewhere between 285 - 300, with peak turbo boost around 18PSI. I've never had a problem with the transmission or engine since. I've probably put close to 40,000 miles on the car since the upgrades. I've basically continued with the normal volvo maintenance schedule, except that I did have the transmission fluid changed as a precaution at 100,000 miles. I'll probably change it again at 150,000, as it's cheap insurance and a good way to tell if anything is starting to fail in the tranny.

I'd say as long as you don't abuse the car, your upgrades should be fine!