You can still stick with Mobil 1, just use a better Mobil 1 than the 5w30 (probably their weakest oil). The Mobil 1 Extended Performance is good stuff, the Mobil 1 High Mileage is great stuff. The Mobil 1 0w-40 Euro is phenominal stuff. If you're pushing 250 ponies, I'd go with the latter.
PU is no doubt going to turn out to be an awesome oil though, so I can't blame you there if you switch. I base this on countless Pennzoil Platinum used oil analysis that are all great. Since PU is going to be their higher end product, it will be better than PP obviously, and that is SAYING something!
Still, I don't think Mobil 1 is getting a fair shake here. Though I do agree, I would move away from the 5w30 formulation. I would run the Mobil 1 0w30 before that, which is a great oil, but I'd want something thicker than that in your app. Much like my fully bolted MS3, I bet Rotella T Syn would turn in great used oil analysis for you. They turn in awesome ones over on the mazdaspeed forums. My picks...
**GC w/ P1 filter combo at PepBoys on sale $27
**Rotella T Syn 5w-40 at Walmart $18, buy a filter of your choice
**Mobil 1 0w-40 Euro w/ Mobil 1 Extended Performance filter at AAP sale $27
Not necessarily in that order. Any of these would work great for you I think.