I have run Fetish from day 1. I logged into EQ2 on a trial account, said wow this UI looks like crap... bad enough that I wasn't going to continue playing. Then a friend pointed me towards Fetish and I haven't looked back since. I have added a few of Mother's pieces as well, especially the new Timer window which I use only when raiding.
Anyhow all that said, in order to get the most out of my toon on a raid and in order to best be able to lead raids I have to raid on Extreme Quality, even though when I play outside of raiding I run High Quality with 30+ FPS which is good enough for me. But there are SO many others factors that affect the lag and frame rate in raid zones that I don't think it would be possible to "test" ui against ui. I don't know this for fact but I am pretty sure that the servers the raid zones run on arn't the same as the servers the "overland" zones run on. When you get 3-4 guilds all zoning into a raid zone at the roughly the same time I can tell a HUGE difference in lag etc. What more I think some of the raid zones share servers with instanced zones... lke PPR that zone lagged regardless of what settings I used or didn't use.
Well, the point being... use the UI thats going to make the game p[ay the best for you. The performance difference in the different UI's in reality, when one considers ALL of the variables that come into play, is so small it doesn't matter. I do realize that not everyone can go buy a super gaming rig (I know I can't) and play on Extreme Quality all the time, but seriously... the UI is how we as players are able to interact with the game. In the long run thats a lot more important to me then being able to raid on high graphic settings
And if you don't believe me, next time your raiding hit f10 twice so you have no UI and see how that game play experience goes... jk