But for me it's really not worth switching from qwerty, as I don't want to have problems typing on someone else's qwerty keyboard when I need to -- which is a problem I've heard many non-qwerty typists report having, even if they were once fluent in qwerty. Some do report that they can make the cognitive switch from one to the other relatively easily, but some can't.
So for me it's not worth the risk of spending all that time to learn another layout just to find it hampers my qwerty-typing ability when I might need it.
If I was going to learn anything, it would be steno -- as at least it being on a radically different keyboard should mean that learning it shouldn't impact my qwerty-typing ability.
As far as say Dvorak or colemak gaining you faster speeds, I'm pretty doubtful of that claim. It has more to do with how much effort you decide to put in.
In that context, it’s a very significant improvement over QWERTY, in particular by avoiding the bottom row, which on a typewriter is slow and awkward to reach. 15-20 WPM advantage sounds pretty plausible to me.
On a “modern” mostly flat keyboard, some other layout would be better than either QWERTY or Dvorak (personally I don’t think any of the popular layouts are properly optimized for common current keyboards, but to be fair the modern keyboard is a pretty terrible design). Dvorak is still a significant improvement over QWERTY though.