TBH, Your not going to get much more speed from changing to HH than you are by implementing my MMU hack. Your probably talking a few fps at best. The only thing slowing programs down now is the task switching, which occurs every 50ms or so. As long as you don't have anything running in the background, Linux should be quite efficient.
Of course, the hack needs a few tweaks to bring out the best in Linux - cache flushing & locking, interrupt handling, etc (Yes, it's possible to get direct access to the interrupts under Linux, I've just not written something thats publishable yet).
Also, before anyone asks, yes, it's also possible to write a MMU-based GBA emulator under Linux. But that's a big job, so don't expect someone to do it over a weekend.