X-Moto is a very fun game and I have it on my "Very challenging ports list". It would take a lot of work to bring it to a 320x240 handheld, mainly because of the GUI not being designed for such a low resolution. I will probably attempt it, eventually.
I am fairly certain that with a lot of optimization in the GL->OpenGLES conversion, good speeds could be achieved. I base this on running the game on my old laptop with crappy integrated graphics and the CPU underclocked to 600mhz, where I still had good framerates. If there is much 64-bit float math in the code, it would gain a lot of speed from being converted to 32-bit float math.