Volvo uses dished pistons to lower the compression in the B21FT - a non-turbo block or flat-top pistons in a B21FT will raise compression but to avoid detonation the I think the boost would have to be kept relatively low or a knock sensor system like the Saab APC, & perhaps even water injection installed to prevent disaster if you want to push the envelope. I think TurboBricks has a few articles on this conversion, or at least on adding turbos to NA engines. As ever, how far you go is ultimately dictated by when the rod exits the side of the block. By the way, I believe the B21FT had beefier rods than the regular B21A or F - my understanding was the bottom end was strengthened somewhat. My old B21FT breathes more happily now with a few more basic mods - intercooler, airbox opened up a bit & an intake manifold off a B230 (LH intake). I might try flat-top pistons some time but seems like a lot of work for a probable sacrifice of driveability (& reliability!).
-Chris