I personally think it was built to remain reliable and within the same bhp bracket as it's rivals.
At the time the E90 M3 4.0 produced 400bhp, the RS4 4.2 about 420bhp (claimed before they coked up and were slower than death) and the C63 about 450bhp from a 6.3 (which going on the theory of a holding back an engine is probably the worst culprit and even in uprated form for the SLS had just over 500bhp - I think)
In terms of tuning your ISF. If the juice is worth the squeeze then go for it. Just don't break it.