Oh no, not the impossible turn topic. You'll be lucky to survive this (the topic, not the turn). I posted something about trying it a couple of years ago and was lucky to escape with my life (again, the post, not the flying).
Theory says a 45 degree bank is optimal. I did it successfully down to 500 AGL using a 45 degree bank but flying about 10 knots faster than optimum (which is JUST above the accelerated stall speed).
Subsequently I tried it at altitude flying at the optimal speed and concluded that it SHOULD be possible from 400 feet as long as you keep the speed and bank angle under tight control. But that doesn't mean I'm prepared to try it for real.
One interesting thing I found is that the altitude loss in the 270 degree turn at stall+10 is the SAME as at stall-5. At the latter speed the aircraft is REALLY telling you how unhappy it is, but nothing bad happens (in my TR182 - I can't speak for any other type or even aircraft). In other words if you do try to fly it optimally you won't just crash and burn because you get a couple of knots slow. Assuming you recognise the stall symptoms of course.
Not recommending anyone go fly this thing - common sense says that if you haven't practised it, land straight ahead. And even if you have unless the only thing you can find straight ahead (or close to it) is schools, kitten farms etc.