It is hard to tell, but I would have expected almost 100 hp more, and a higher RPM torque peak. If it would have made 900 hp STP before the inertia correction, that would be acceptable.
The headers are a touch small for a 572 (2-1/8"), and will reduce peak HP. But the Enderle TR intake should be good for power.

My guess (repeat just a guess) is the heads are just not very good.
Even Wade's 508 CID with 12.5 CR and "only" Koffel's stage 1 porting made similar power to yours. But with the expected peak torque RPM.
Koffel says 390 cfm, but IMO their stage 1 isn't a big power producer, but may be fine for 508 CID.