You take away his turbo, leave the motor alone with turbo manifolds as exhaust and his car wouldn't make anything CLOSE to 500whp. It's an ls9 with 9:1 compression, with the motor exactly how it is without the turbo it'd be lucky to make 370 whp.....adding 4psi will not yield 250whp! And yes I bumped this page because I came across it on google. I wasn't going to search for an updated one to comment on.
FWIW, my 5.3L (stock bottom end) with nominally higher compression, good, but not as good of heads and some really mediocre shorty headers did >400 RWHP. I would think that properly setup Mike's combination should do better than that N/A.
Also realize that not all engines N/A vs. turbo behavior is a linear relationship of boost and power.
Again, I'm not saying that Mike couldn't have had a touch more boost, but 600 RWHP at 4 psi isn't really that shocking to me. BeasTT's twin turbo car does over 600 RWHP on 6-7 psi all day long, and he's got hand ported heads on a 9:1 6.0 block. It did ~1000 RWHP on 17 psi without everything really dialed in yet.