Wiki says the most powerful NA I6 cylinder engine is the 405 hp Speed Six from TVR, the most powerful NA flat 6 I can find is the mill in the current RS, and I doubt that there are any naturally aspirated V6s that are more powerful than either of those.

If this super GT3 is made, then it almost certainly would be the most powerful naturally aspirated 6 cylinder fitted to a road car.

In researching, I found this tidbit. It's a post on a GM forum that compares the engines of a G6 GTP, and an NSX. It is completely missing the point. Also, one of those cars is a legend - the other is a piece of shit.