Well. as far as the gearing ratio is concerned seeing as the 750 motor comes out of a 4spd auto that will tell you something compared to the tip.
now as far as how much the boxster tranny can hold. I honestly am not sure but give a call to the guys at GBOX they if anyone will know. I believe they told me that their specially treated race tranny could hold like 650 or something. and to me if theirs is an upgraded stock boxster tranny I don't see how the stock could handle more than that. to be honest I cant imagine a passat tranny holding up to 700hp either.
it will be an undertaking to be sure. now how will you fit this motor? being Monocoque is there really a way to expand the engine compartment in a feasible way?