The only way to be sure how the particular formulation used on your machine will react is to try it - remember, this things were built over a period of a few years by a couple different manufacturers. The plastics, even if spec'd identically, will have batch variations. Each machine was exposed to different conditions over the course of its life and will yellow more or less than any other machine.
So, having said that, I'd suggest you do an experiment. Do a small square of the very bottom of the case and see how it goes. Last weekend I de-yellowed one of my 1200XL's which was dark yellow, tending toward brown. It's now nearly pristine white again, much like the photos in all the XL magazine ads and catalogs. But XE and XEGS machines were never purely white; as you say, they were gray. So I'd suggest you spend an hour or so and read the very long pinned thread in this section and pay close attention to the XE-specific posts. Very recently, Jon (flashjazzcat) resorted to painting an XE computer and it looks quite fantastic.. However, he and others have successfully used 40 Vol Cream hair peroxide products on other XE machines.
And use the clear on the colored buttons as well?