That is especially true of Dvorak, seeing as much of the 'original research' by Dvorak himself was conveniently lost. I agree most gains are 'marginal' but some are still larger gains than others. Personally, Dvorak did nothing for me - and most gains I see people making is because by learning a new keyboard layout
they finally learned how to type which is where most of the benefits actually come from. It doesn't matter what layout you switch to, switching to any new layout and
learning to type will improve your speed and comfort.
I knew how to type in QWERTY (140-150wpm) and switched to Dvorak where I quickly got up to speed (130-140wpm) but found very little improvement by any other measure. So I switched to Colemak, which I find far more comfortable than QWERTY and I'm capable of the same speed (140-150wpm).
My biggest problem with using Colemak is nobody else does and I'm not going to convince everyone to relearn how to type. So I use Colemak at home and QWERTY at work.