Photoshop users use Mac because it works the best for what they're doing. If they could have had the same solution vastly cheaper and 80-90% as good you think they'd have been willing to pay thousands more for that 10-20%?
Photoshop gives you much more than 20% over GIMP. Photoshop is light years ahead of anything else out there and only getting more so (see latest sneak of deblurring photos).
