My amigaos build is original dosbox + all your ppc patches + sdl2 patch (so sdl2 swith surely may have impact) + cdrom based part from dosbox-staging as it support audio from cd better than in original.
But, to add on that, all lowend custom ppc amiga hardware are crap if we compare it with even old ppc macs or wii, as it done by entusiast in low quanity, and mostly for labor of love than business. As such all the things related to cache's speeds, speed of memory tranfer most of time suck.
Add on top that it controlled by AmigaOS4: not the OS writen by genius, but just additional components and improvements at top of old, BCPL language based, AmigaOS3 from begining of 199x era. There even still some 68k code left which is internally emulated.
So sdl2 patch, custom slow ppc hw, amigaos4 : nothing surprised. And i do not know what samo expect at all with his hardware: stuff which works slow for him, will continue works slow. Be in hope that anyone will spend hours because "i want 1 fps" to have the same slow speed i see no sense.
My PPC machine from othet side are 2ghz ppc and comparable at least by speed with latest ppc macs, and dosbox works good with your jit.
Sure if there possibility to speed things up for cheap, that will be cool, but loosing a lot of time for +0.1 fps make no sense.
But for real, what is need it in terms of ppc support is 3dfx . That surely more important than improve benchmarks on 0.1 fps.