I'm not sure of this, but the DSTwo sports 16MB of RAM, so games big less than 16MBs are most certainly entirely loaded in RAM.
Access speeds might benefit large games like MOTHER3, but I'm still pretty sure that GBA is slow because it's emulated (and the DSTwo isn't that strong to start with) and not because of access speeds.
tl;dr, buy one if you find it cheap, but keep in mind that there's a HIGH chance it won't improve GBA speed.