when changing exhaust is it really necessary to disconnect the battery to allow it to "learn" the new air flow?

 

you could just do a computer reset in the test menu, thats all disconnecting the battery does.

 

Never heard of needing to reset or disconnect battery for that reason when changing exhausts on a car. The computer will work with current conditions and acclimatize as those change as far as I know. On the other hand, if you really want to optimize changes (I'd say more then just the exhaust, but other mods as well) an aftermarket software upgrade can help.