It depends how much you over do it. If it has like 1-2 spells active at the time ,you dont really lose any FPS. But if more than 3 are active at the time you will lose some regardless of what PC specs you have.

Now obviously it makes a difference when you have a good PC and gonna lose 10 fps you not gonna notice, because you already have 100+ something, while on a low end if you are at 40-50 you gonna feel that 10-15 frames drop.

You can always make a normal and light version :)

I personally already did something similar but with actual spell animations with the rogue crest, like when I'm full combo points the eyes of the emblem lit up or combo point pointers have a shadowy-wave like effect.

If I could draw like you would make amazing artwork for an entire rogue UI.

Would make WA unitframes with badass UF borders and syles and bar borders, something like an entire theme (ex: http://www.wowinterface.com/downloads/info24458-DiabolicUIGoldpawsDiabloUI.html)

But I can't draw shit :D