Remember, it's not all about peak horsepower. One of the strengths of the VQ is its constant torque curve. On the track, engine response (having power/torque within a broad rpm range) is what its all about.
btw...the thread starter on this thread on maxima.org:
http://forums.maxima.org/showthread.php?t=334712
he was a pioneer in achieving high hp #s for stock interals on a vq30de.
and if you check...550-600fwhp is the consensus...