Reliably, keep it under 300hp and you won't grenade the tranny. That's from a VERY reliable source.
We're running the straight IPD ECU upgrade with exhaust which should be putting us in the 290ish area. It cuts back the torque around shiftpoints ( I think this is common practice for chips on these cars, but it's a nice touch). I've seen people do stuff like wastegate mods, but since I haven't done it, I can't speak for the reliability factor with that. I do know it'll give you better off the line performance. I learned that first hand when I raced an 850 turbo (Prospero's) who had it done and a mystery chip. He pulled on me initially and then we hung together. It's late and I'm going on and on, so I hope this helps.