Does slip on cans void the warranty on a new M659?
I have also read that no ECU tuning it needed for slip on's, true?
not sure what an M659 is...so no clue...so I think it best to consult your dealer....
In the US aftermarket parts do not void a warranty unless they are the cause of the problem.
depends on the slip-ons. some are designed to run with stock ECU settings (with the baffles in)
there are performance advantages to tuning, but the bike may run fine without as well
Warranty laws are very specific to the countries. For instance in Chile the dealers provide a full warranty as long as all maintenance is done through the dealer. So they would only provide warranties if they install any mods.