Olaf, I've got a little tip! It took me ages to discover it myself but one day it just hit me! I own a GO 139 and from the beginning i've set the straps fully outboard (dunno why, jsut wanted to make my life difficult i guess ..) but after a while i had the same problem: go fast, no problem, controle the board? uhm yeah ... like barely, and thats cuz i weigh like 95 kilo. Simply cuz i could't cruize in the back footstrap; i was always in the front one, no problem, but the board went ballistic when i tried to get into the rear one. So one day i gave up and said: if i cant get into it, i can try to get behind it: that way i can controle the board by adding a bit of backfoot pressure and because i'm one step closer to the tail, i'm more on the fin and i gain some controle. It worked perfectly and once i was planning i said to myself: if i can get behind it, i can get in it. I tried and what do you know, it worked. See, if you're behind it, you're basicly in an exagerated stance and if the board can tollerate that, it can tollerate your foot in the rear strap for sure. Nowadays on the GO i still opperate like that. Only on my Flow, which has more inboard setting (which frankly are getting really boring to a point of annoyance) i get in the footstraps without going behind them first.
once you get into the rear straps, that board will accelerate beyond what you've ever felt before! Just be carefull not to catapult
at least not when you're hooked in, i swear you'll break a boom or a sail (minimum!).
I hope that helps muchacho!