But he has tried a lot of different combinations of pistons and bore sizes so he may be able to give you an idea of what works well on the track and what doesn't. If I am remembering correctly he is now running a pretty stock compression setup with the idea to go forged internals to address some reliability concerns.
If it were up to me I wouldn't stray too far from the stock block and address the powerband with a medium sized turbo like the GT28 or GT30 if you want more power up top. This would allow you to fix the car with off the shelf Saab parts rather than needing an expensive rebuild should anything happen to a fully built up 2.5 monster.
I know its apples and oranges but I hated how my 900 drove with the lower compression of a 2.1 head/manifold, now I'll bet it could be remedied today with the better ignition control of T5, but it was noticeably slower down low. I thought the stock B202 was more like 9.0:1, I prefer the slightly higher (9.3:1 I think?) compression of the B235 I drive every day, that combined with a TD04 gives a very flat powerband. I think the same TD04 out of the 9000 Aero with a larger compressor wheel would be a fantastic combination in a c900.
