Hi Steve, you're right, mod-DH is a big improvement over vanilla colemak for the 'HE' bigram, I might have understated that. However, making it a ring -> middle finger roll instead is an even bigger improvement, and I think that's important for the second most common bigram in English.
I would also say that I didn't really choose to put all vowels together, that was the work of the algorithm, as soon as you value lowering outwards rolls, the A key has to move from its qwerty/colemak position.
In terms of swapping I and A on my layout, it actually leads to more single finger bigrams (depending a little on the corpus used) -

As well as a lower inroll : outroll ratio. This layout is heavily optimised, you can't really improve the single finger stat without a significant rework. Colemak is very optimised for single finger, so I knew I wouldn't beat it on that stat, but you can see that my layout is not too much worse on this, its other qualities make up for that to me.
edit - also, swapping I and A would make 'IF' a single finger bigram, which, as a coder, would not be fun at all
