actually no exhaust system from 2011 and back would fit very easily on the 2012s. 2012s are a single exhaust piece of the header back and i think they have different hangers. So you would need to not only cut and weld the pipe on, also fabricate new hangers.
And "would drastically changing the exhausts system on the 2012 model require a power commander, or would the bike be fine with out it?" is not a big question at all. It would require a power commander just as much as a previous model would. You dont require one, but you will get better gains with one.
2012 Ninja 1000
2009 Ninja 650 - Stolen