In terms of ISO, is the ISO performance compromised? Say I was using a 50mm DX on my FX body, would I have a lower quality ISO performance vs an FX 50mm on my FX body?
Well, sort of. If you use only part of the sensor — as you do in DX crop mode — you are inherently gathering less light. (The light per area is the same — so your exposure settings are the same — but you have less area.) That means that if you print at the same size as you would an image from an FX lens, your print inherently has less light per area — or, to think of it another way, any noise would be enlarged more than it would be otherwise, and theoretically more apparent.
Of course, this is also true if you crop some area from an image from your FX lens and enlarge. In most practical terms, the difference is so small that no one ever thinks about it.