NA engines are always better, they are much crisper and you don't get the turbo lag, also the power delivery is very smooth.
That thing already has enough power, didn't you see the Top Gear test where they could hardly keep it on the track?
Nice car though, I wonder how much more power Brabus could get out of it.
http://www.youtube.com/watch?v=5eOLq07VwSE