Well i have tried it out, it is clunky.
Steps I followed.
Grabbed dev mode activation from store.
Signed up to dev account (costs £12).
Ran the app to activate on dev account.
Took 3 attempts to activate successfully, but 3rd time lucky.
App then told me to wait for a update to download, which never did.
There is a hidden dev menu on console you can activate on the console info screen which I did.
From there I enabled developer mode and rebooted it.
It took eight minutes to reboot, thought I bricked it.
Then from there I had to configure wifi again in settings, and added my account.
I was able to via the web page portal to upload retroarch, but this is really clunky and it kpept getting stuck and when it did work I forgot to add the library dependencies. So I uninstalled it.
Couldnt reinstall, it was getting very scuffed in web UI as well so rebooted it.
Took another three minutes to reboot.
This time i was able to properly install retroarch and it launched fine, at this point test was done, didnt bother with any games.
Selected leave dev mode to go back into retail mode and this took another 3-5 mins.
After back in retail mode I did a manual reboot, and thankfully was fast. So I havent destined my console to 5+ minute bootup's in retail mode.
With all this done I have observed 24 gig space is now used extra on internal storage, so there is a 24 gig internal storage tax to using dev mode, I expect the slow bootups might be some moving data around on a new partition or something.
Apparently to remove dev mode you have to factory reset. So I am stuck with this 24 gig usage now unless I am prepared to factory reset. However if I get round to installing games on to retroarch I would maybe consider it worth it. Currently I have retropie for emulation, which works ok. But can struggle in heavier games, and adds more cabling mess with it been another device.
I also got the above working and agree it's clunky - above instructions spot on but there are some good videos on the Tube. I've since managed to get added to a 'developer whitelist' for using the same app on retail xbox so more convenient (no developer access required). You can even play stuff directly off the internal SSD. Incredible that it works but not sure how much I will actually use it.