300whp is common on what I've seen. Most people dont really push the envelope with that engine from what I've been able to find. But, most bmw i-6 engines can make 600-800whp with only a thick HG. The myth that stock internals are weak is just that, a myth.
Yes, you're right nukeduster on the thicker head gasket. That's definitely something that is overlooked. I would also look into some heavy duty head studs and bolts (it can only help prevent lift of the head off of the block). The real key here is tuning everything so it works correctly and safely.
You'll be happy to make 240 rwhp. Don't be stupid, and don't push it beyond 8 pounds. Where do you live? Everything here has to do with tuning. Since you're asking such a vague question I am willing to bet you have spent anywhere from 0-1.5 minutes ever talking to someone who knows how to tune your car.