I only tested on the 2.08 exes that Duzenko made available to test, and on those it works, I see no visual difference (with one exception, see below...) nor performance difference with normal map compression 1 or 2, what I see is a huge reduction in GPU memory usage, with it off the scene I was on was using 1389 MB of VRAM with with on either on 1 or 2 it fall down to 878 MB, that's the difference between a older 1GB GPU being able to play the game well or not, on mine GPU that has 8GB of VRAM I could, right now, very well play with compression off.
... with RGTC enabled the water in Volta 2, at the beginning, looks even more broken, now it shows a reflection with the colors totally messed up, man that water shader does need to be reworked.
p.s I tested on 2.07 now and it worked, what i did was start Volta 2 went in-game and only there change the svar, on the menu I saw no difference, I assume that most be because the menu does not use any normal maps.