At some point (maybe Unity 3?), GrabPass changed and it no longer seems to take any of the arguments it once did:
And there are some corresponding globals that are no longer filled out. Does this mean that GrabPass is always the whole screen, and always at full resolution? Does anyone know why this might have changed?
Sorry, I don’t know but I am very interested about the answer !
I miss the answer too, where is Aras?
I am interested in the answer as well 
Oh yeah sorry guys I talked to Aras about this last year. He said those options don’t exist as of Unity 3, because all GrabPasses are full screen now.
If you’re writing for mobile, though, GrabPass is insanely expensive and not a good idea ever. You’re much better off rendering what you need to a texture and feeding that to your shader manually.
Okay thanks for the reply !
I know GrabPass was expensive but I didn’t know it was slower than a RenderToTexture !
I’ll try that et see if that fits my purposes.
probably due to it being tile deferred or whatever. I imagine grabpass isn’t much different on the new tegra for instance.