It actually takes working against the way Enyo works to seriously break compatibility with multiple screen sizes. I've had GVoice running on anything from a Pre-, to an iPod Touch, to TouchPad, to iPad, and some of my friends with Android tabs that are 1280 res, as well. I did have to design a quite different interface for the phones, but it was actually quite easy to do so, and have the interface adapt completely to the size of the display, as to whether it used a tab-based interface, or tablet style.
If I actually knew CSS, I could probably get it to be even slicker.
I don't think that we're necessarily going to see all the problems that you're thinking of, performance wise. I have a feeling that some work is being done on some pretty low end equipment, and I have a strong feeling that at least some things about Open webOS are going to surprise us in a very good way.
I've never been known for being an optimist, either.