RMVX's Engine is no different than XP. A few of the classes in the RGSS Library were updated (Graphics for example) and through that, and dumbing down the Tileset related stuff, it made for more improved frame-rates.
RMVX is neither an improved version of XP, nor a side-grade. It's a very different maker, and is designed to make very different games by default. Through scripting, it can make anything, just like XP.
I was an advocate of RMVX, but after seeing how difficult it is to make a 'Mountain Range' map, I have given up on it. If I decide to make a game that doesn't use the tile engine, I will definitely go back to it and use it, but it can't make the RPGs as visually impressive as XP by only using the default tileset editor.
To explain, the issue comes with the way the Autotiles work. You can't make two platforms beside each other that aren't connected, because the autotiles automatically try to connect. And because there is no directional passability, even shift+clicking the autotiles to get it to look right won't work because you can still walk between them. Also, the wall-faces try to blend together, rather than recognizing undulation properly. It's a tricky system to work with, but overall doesn't produce what I want it to. It's a shame, though, because RMVX's RGSS2 Library is just a treat to work with.