Its a noticeably faster CPU than the XP8. Unless you need all the extra relays, ports etc. i think that you would be fine with the XP-3 if you are only adding a few IP drivers. the biggest concern is how much Memory your project will need.. I don't believe that the XP3 has expandable memory (ive yet to use one) and is limited to the 128 Megs on board which is probably (I'm guessing of course) enough. The biggest hogs in any project as far as memory is concerned is going to bet the GUI for iPads, iPhones, Android etc..Here is a recent project image of the memory consumption with some decently hefty iPad and iPhone GUI.
Depending on the size of the project, the XP-3 may be the way to go.

[Link: ibb.co]