Your 603 can easily be modified with ALDA adjustment and wastegate adjustment for more fuel & 15psi boost from I believe 10.5 stock, to go above that you can start to adjust the rack and add a bigger turbo, add an intercooler, it's all on here somewhere.

There's a smokey-donut video around here somewhere of a couple of cars (W124 and W201) that a Dutch firm tuned pretty extremely.

- Jeff