Performance Data from WT
Deriving performance data from WT data is possible, but it is unreliable. Big Aircraft manufacturers (A... and B...) do it (for projects of a new aircraft, for example), but still they apply quite big factors just to be on the safe side. To do the same with a GA aircraft and achieve high precision will probably cost more than building the whole thing, then flying it....
I agree with Genghis' ideas, but don't use TORR. It is awfully difficult to predict, with all those dynamics, ground effect and stuff going on all at the same time.