dont quote me on this but i dont really think it would help the only thing differant is that lite doesnt include the 20 man raid the 25 man raid and the 40 man raid unit frame panels so its lite in the fact that its lite on unit frame panels? lol idk so yeah i dont honestly believe it will help your lagg or FPS at all but i could be wrong.
EDIT: i think that in my nUI lifetime i've asked for the HUD seperate 3 or 4 times
so have a bunch of others but scott likes to keep it all together and even if the HUD was seperate who knows what else it relies on to keep itself running if it was alone it might lose some features and stuff so you might not want it seperate even if you could have it seperate