There is a modification that you can do. It is sanding down the wings. If you take apart the 5x5, and take a wing edge and a + center, and put move the pieces by each other, you will see that part of the wing is too thick, and resists the + center from sliding by a bit.
The problem that I have had when doing this, is that I didn't do a good enough job of preserving the "round-ness" of the wing, and so the cube now has a rattling sound with it. Well, actually I have attempted this twice now.. the first was horrible, and the x centers were twisting machines, so I threw those pieces away. The second was better, but still not like Grant Tregay's cube. (He found the mod)
I don't know. I think that just working the cube in is the best bet. Either that or what until a better product is out. I heard that a Rubik's Studio 5x5x5 is on the way with this mod already done. Check on twistypuzzles.com for that info.