The P heads definitely make the power. They seem to be the latest rage and even I'm putting together a set to replace my old irons. The real benefit is in the smaller combustion chamber. You can basically push the compression ratio as high as you'd like it (within reason). Compression ratios of 10:1 are achievable with mild milling.
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Jeff Chambers
T/S #4
12.40 Seconds/109.5 MPH
http://www.kellnet.com/chambers/default.htm