Pretty accurate in most cases I suspect. That cruising speed is at 28,000 ft, and I haven't been that high in it yet, but it's rated at 290 ktas at the high teens where I have been and from memory overspeed at that altitude kicks in just above 300 ktas.
Edit: - Well, I'm going to take that back, at least as far as the TBM is concerned. Thought I'd try it out, climbed to 28,000 ft and put throttle to full (which led to TRQ warning lights and pressure differential warniings!), but couldn't accelerate much above 220 ktas, and the overspeed warning was still showing on the speed tape, so something amiss there.
I will say that I've not been able to feather the prop in the TBM, and I don't know whether you need to adjust mixture in a turboprop, but even so, it seems as though it isn't modeled accurately.
Of course, I may be doing lots of things incorrectly.