I can speak from what I've seen running in competition. Nearly everyone (90% or more) running competitively in T/S has switched to the P head because it just flat-out makes more power. Flow numbers aren't everything. The combustion chamber in the P's is superior, the exhaust port is superior, the spark plug placement is superior, and the deck thickness is superior, just to name a few.
We run the heads un-ported with a small (.500") cam, either carbed or EFI, and we've got guys making 340-370RWHP. You do the math, that means 425+ at the crank. Not a small feat. That's with shorty headers and stock sized valves too. I've had both GT-40 and GT-40P heads prepped basically the same way (gasket matched to 1250 and 1415 gaskets) and even though the flow numbers were very close, the P's make abundantly more HP. I think their key is runner velocity...they absolutely love it and its what makes the small valves work.
Don't discount them if you're in the market for iron heads. With a nearly endless supply of them at $200 a pair (bare), they're a very economical HP bolt-on.
Look at the guys running in Pure Street too. Tom Payne set the record last year at 11.09@122+ with a set of P heads.
Just my $0.05.
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Jeff Chambers
Trophy Stock #3
11.611 Seconds / 116.7 MPH
Chambers Racing Team