We've gamed from 1st to around 8th level, and several one-shots using characters all the way to 20th. M20 definitely shines at lower level, and while it works just fine for very high level stuff, the general player feeling is that D&D does higher-level better.
I think there reaches a breakpoint where players want the cool-but-complex funky stuff that D&D brings to the table, and M20 just doesn't provide the same level of meaty crunchiness. It boils down to your style of play, though. If you want legendary characters who are extraordinarily capable at what they do, but don't rely on magic/wushu/items to do it, then M20 is very, very good at simulating it.
I'm thinking Waylander or Conan here. Conan would be a terrific 20th level M20 Fighter
On the other hand, if you want a "classic" high-level D&D character complete with a feats, special abilities and a direct line to several gods, look elsewhere. At D&D itself, say.
As you're aiming a little lower around the 12th to 15th mark, I don't think you'll have any problems. Just toss around a few ludicrous skill penalties and encourage the players to be cinematic with their actions. In one 15th level playtest I had a player run up the spine of an enraged Red Dragon by making a Phys+DEX check with a -15 penalty. He did it too, but that's another story.
My vote would be to keep a weather-eye out to how the game plays and if the players start to grumble about the lack of options, consider migrating the characters to D&D. You never know, it might never happen
