I agree with GFD about the .12 and better fuel mileage. Especially if you run longer mains at your track. I like being able to pit later than everyone else. It takes the pressure off your pit guy and makes it a lot easier to deal with pit traffic.

I think the Pro-spec will handle the horsepower, I just think it might be overkill anyway. Trust me, and RB V12 5 port will run as well as your X12 if not better. There is already enough power there. Too much power is just a waste anyway unless your track is giant and you can really use it. I couldn't begin to tell you where to gear an .18-TZ because I've never ran one before. Haven't had one on a dyno either. Gearing is subjective on every track anyway. Our track has a different layout from race to race and I sometimes have to change gearing just to accomodate that.