As far as the poll, which engine is better for big HP... the US-spec DE is probably the best, believe it or not, because of cost. If cost is not an issue *at all*, then the GTiR motor is better.
For the extra money it costs to buy a GTiR motor over a spare DE, you could build the DE up with forged pistons and rods and put piston coolers in and probably some other goodies. At that point, you're in a position to spend far less money on finishing it up than you would be if you're doing a GTiR, because even with a GTiR motor you'll have to do forged pistons and rods, and headwork if you really want to do it right.
I'd say the GTiR motor is the best and most cost effective way to go fast *IF* you leave it stock or close to it. Once you start replacing internal parts, it would have been on average $1500 cheaper to just do that to a regular DE, and that cost difference alone would pay for the upgraded parts.