[Easy Rider 2]

You do not require a power commander or ECU flash when using a slip on. No power will increase or decrease for you. You should use an ECU flash rather than a power commander if you purchase a complete system (i.e., headers).

drift boss​

Well that's not necessarily true.
It depends on the specific vehicle design.
IF you are removing a "real" muffler and replacing it with an un-baffled can, then there could be some impact on the fuel handling. Especially if you open up the intake at the same time.