RE: evo xtv 70
OK, at 83 kilos I don't think you will be planing earlier on an EVO 70 with a 5.7 than you will with a 5.3. As you say, there can be instances when a 5.7 can still offer some benefits, but even in such conditions there wil be donwsides of such a big sail on a small board.
My suggestion would rather be to look into getting an extra powerful 5.3 sail which can be trimmed for lots of bottom en grunt and that also is good for pumping.
But if you allready have the 5.7 you should just give that sail a go on the 70 and see for yourself how you like it.