Drrc, I can tell you the p5 that Monty(HES) sent me in this Mississippi weather all I had to do was lean the high speed by 1 hour that's it. So in my experience he's pretty darn close to a race tune. As far as the n21 vs. Plus 4 arguement, I own both and to me they are 2 totally different engines. For the money the n21 is by far the best engine for your dollar. It's more of a pull the throttle and here's the power, whereas the plus 4 has a very smooth power band across the board. Again this is just my personal experiences and if money wasn't an issue I'd go ahead and get the plus 4 but you can't go wrong with the n21 limited. Now, I'll let Monty step in and tell you the right answer to your questions...lol