I just have the standard version of Rx but here's my shot at it.
If the advanced version has true upsampling (not just filling it in with zeros) than, just like some of the better equalizers which upsample before it's applied (smoothing out the curves) I would imagine that Rx does the same thing and for the same reason (just a wild guess here). If this is the case, you'd only notice a difference when it wasn't being upsampled and it needed to be.
To find out the real poop on it, why don't you just email iZotope. They have guys there to answer questions like this and if you paid for your software, you've already paid your share of their salaries. Why not get your moneys worth?