Those numbers are really good. If that L99 had long tube headers and high flow cats one would expect it to gain another 20-30 rwhp. That would put that L99 in the 380 - 390 rwhp range.
That's seems almost too good to be true.
Are his numbers in line with what the other stock L99's are pulling on your dyno? Because I'm trying to see if I can find a relationship between temperature and humidity and lower stock dyno numbers. I'm finding that high humidity combined with hot temperatures are negatively affecting some of the stock L99's.
So far the three highest stock L99 dyno numbers I've seen have come from Northern states (N.Y.and Michigan).