Maybe. The 360 has a tiny bit of free space between the plastic and rubber membrane, so putting a spacer in there makes a noticeable difference. The FX pad (and probably the RX pad, if they ARE the same), doesn't really have any free space. You could put a paper-thin spacer in there if you wanted too, and it might do something, but it could f*ck things up as well. The rubber domes will be constantly pressed down and you'll end up hitting directions by accident.
Seriously though, doesn't anyone have pics of the inside of an RX controller? A couple of you guys have them, so could someone take it apart and take a look at it? Taking pics would be better, but at the very least we should see if the rubber membranes are the same color. That's a good way to see if they used the same parts or not.