If you have a good conservative tune your going to have a bit of exhaust out the back at WOT. A conservative tune is going to be a little on the rich side to keep the EGT's down and keep from blowing holes in the tops of your pistons.... The dark exhaust is unburnt flue and yes while its wasted energy, its also means your not running too lean and there for not making WAY too much heat the leaner the mix (to a point) the better the performance.
