What are you using to overclock your 2215? Immier's custom build of xcpuscalar, or the latest version of PHM? I know Immier's 2.5 release included a 530mhz option that didn't overclock as well as the custom version.
As far as settings are concerned GBA with sound is pipedream at this point. It is better to turn off sound, keep your frame skips to 3, auto sync. Also, enable the debug option in Global settings to see your frame rate. ROM size shouldn't matter. Certain games will get pretty decent frame rates because of the type of mode that is being utilized.
Morphgear is not crap, the only problem is Aaron's choice of emu cores, in particular, the GBA. The SNES core is getting there, and the PC Engine is doing great. He did a lot work on the PC Engine core which he is not charging a dime for so I will not knock his product at all since a good amount of his hard work is free. And like you said, there still isn't another GBA emulator worth mentioning so this is as best as it gets at this point.