arent the SDK's backward compatible ? you can follow the same strategy as you do for desktop jPCT. ie: target a reasonable lowest version and additional features may depend on running version
emulators support all versions i suppose. if your concern is real life testing, community would do it
for performance and memory: i dont think google would let go Android, possibly it would be in market for a long time. in time new phones' performace and memory wil increase for sure.
last month i've joined a seminar in a university about game development. one of the speakers was a turkish guy working at heavy iron studios. someone asked how to decide minimum requirements of a new developing game, he answered: target the highest hardware available on the market. minimum time for a game development is 1-2 years, and after that much time the highest hardware of now will be average. i suppose same applies here
anyway, it's your engine and your call. but if you port it to android, i will -at least- attempt to make a light karga on top of it..