Yeah, one of the things I feel the d20 Conan game did right was cap out the level system instead of keeping it open ended.
I love this.
Level, really then, is a measure of an avatar's combined skill and experience in a certain type of work. Level 1 is a complete novice. Level 20 is a total master.
I picture most to lie somewhere on the bell curve.
In standard D&D, level is nothing but an arbitrary game statistic. Guards in a 1-4 level dungeon may be level 1 guards, with the sargent at Lvl 4 or 5.
Then, you play a level 8-10 adventure, and the guards are scaled: the same type of guards as in the lower level adventure are Lvl 8, and the sargent is Lvl 10 or 11.
Not so in Conan. Because of the level cap, you've got one scale to apply to all. I like that a 14 year old, just now picking up a sword and learning to fight is a 1st Lvl Soldier, but so is the 45 year old barkeep who's only broken up some bar fights in his life.
The guards in towns are typically the same Lvl everywhere, no matter what the PCs are Lvled at. The Sargents are the same, too.
Plus, the Conan experience system gives the GM supreme power to keep his game balanced, given this Lvl structure in the game.
This is all a very long winded way of saying that, yeah, I agree that Lvl 20 should be very rare (3 standard deviations), and I'm not too sure I'd be that interested in any material discussing that level.
OTOH, though...
This is a heroic fantasy game, and I can think of a situation or two where Lvl 20 epic questing might be a very, very interesting story to tell.
I'd say... It depends on the quality of the material. Some superbly written, excellent Conan game material might get me interested in an extremely high level game. But, if we're talking about the same-o, same-o, then, no, I'll prolly pass.