I run the P5 in my MBX-5 at our track with is fairly long and have no troubles keeping up with any car on the track the P5 is awesome for fuel economy i get up to 10 minutes on a tank not running it lean either. the 421 heaps masses of HP but it is very peaky (another mbx-5 at our track has one.

while i cant say dont get the 421 i can definately recommend the P5 especially with the price difference