I did a google search and found a thread where they mentioned that the most powerful dyno at the BMW factory could deal with 1280hp but the turbo engine exceeded that
So I think that the 1500hp was sort of calculated or guessed. Meaning they look at the engine power curve upto say 1200 hp at whatever revs it was doing at this time and extrapolate the data to the redline.
It seems that BMW was hunting the scrap yards for well weathered 1500 cc blocks as they felt it had suffered all the wear and tear it was going to.
They then propped it with new mechanicals (pistons, valves, cranks, etc) and let it rip.
They felt this was less risky than subjecting a new block to all this power.