Forget about mame core in Retroarch. It's based on mame .174 and is too demanding for GPd XD. Even shield TV (which is the more powerful android device out) cannot handle it properly. For MAMe the best and almost only option is mame4droid 0.139 from google play store. And FBA core from Retroarch for other arcade boards.
If you are interested in Saturn emulation, forget it using your current firmware, since 3.4.7 uses new mali drivers that breaks uoyabause performance. If you don't want to use custom firmware (I have a 1,4 ghz version just like your stock firmware), the best stock firmware is 3.4.2, since is the last one with the best mali drivers. But it's up to you and the things you need to emulate.
As far as I know, GPD won't probably release more firmwares for this device, and me neither. so those are basically your choices.