I don't use a wide mod on the Typematrix as I can't reach the center Enter and Backspace keys with the wide-modded hand. The Typematrix is in general better suited for a wide mod with the right hand, (8 columns on the right side, and only 6 on the left), but unfortuately the right hand is the one I use to reach the center keys and I tend to use the left thumb for the modifier key. Wide modding the left hand is not so nice due to the vertical shift key (a and z are the same key then) and not having anything further out to the left, but I actually might try this just to see.

Still, I am most curious about the degree of stress on the thumbs with Kinesis, especially if using it as a modifier (chording with the same hand).

I also use Colemak on a Kinesis, and no problems at all. I do a lot of C# programming. I remapped the delete thumb button to be esc (for vim), I also used macros to make the [ be a { and ] to be a }   and vice versa - typing a { types a [ for me. I did this so that I don't need to use the shift key when putting the curly braces in my code since they happen wayy more than arrays with the [] syntax... :) For more info on my re-mappings, see this blog post: https://www.gregd.org/2012/10/kinesis-a … remapping/

I also own a typematrix - I use it on my work laptop.... it's nice and very portable, but I like my Kinesis (which is attached to my desktop computer) even more! I think the thumb position is MUCH better on the kinesis, and haven't had any trouble with stress on my thumbs...

I use a pretty vanilla vim setup (I have a number of customizations, but nothing related directly to colemak), and have kept the default mappings for the arrow keys. I usually use the hjkl keys on my kinesis, but do find myself using the physical arrow keys from time to time - whatever happens to feel more comfortable. On other keyboards (typematrix included) I use the default vim mappings and it works well enough for me. this was a conscious decision because I didn't want to have to copy my vimrc to new computers/servers in order to be able to use vim.

I didn't know that you could set up the typematrix to have arrow keys on the hnei keys... how did you do that? I may have to experiment with my kinesis and see if I can come up with something similar... overall, I think vim is better with the colemak layout than in qwerty. even though you loose the hjkl navigation, you get 'n' for searches 'i' for insert and 'o' for a new line all on the home row - which I think is nice.

An update...
Two months of using the Kinesis and I quite like it. It feels super slick and the thumb buttons are everything I hoped for. Unfortunately I am experiencing quite a bit of pain in my right thumb (Enter and Space). I have been doing an unusually heavy amount of programming during these months though. The pains in my fingers and back of my hands has completely gone, so I am very pleased with that. I am trying to not bottom out the Kinesis keys but its a lot more difficult than I thought, especially with the thumbs and modifier keys.
For programming, I am still a bit undecided, but for writing emails and plain text, I am totally in love with Colemak on Kinesis

Great to hear!
You may want to do the O-ring mod (cost $5-$15), that will help with bottoming out by cushioning the landing, and will reduce the amount of travel from 4mm to about 2.5mm (don't worry, the switch registers the keystroke at 2mm).

Another idea is to put Enter under one thumb and Space under the other thumb. Maybe could remap one of them to the Delete key.