Turbo engines for sure. There is a lot of conservative design so you don't blow the head off. You can probably reduce turbo 603 0-60 time by a second with an EGT gauge and tuning. The turbo 606 you can tune with a chip.

If 'a few more hp' is a goal, start with as much factory output as you can get - 17x hp with a turbo 606 or 14x hp with a turbo 603.

Start here - http://www.peachparts.com/shopforum/forumdisplay.php?f=100

Sixto
87 300D