Flashed, tuned, and a power commander is claiming three different things that all do the same thing. It’s a matter of where and how the tweaking has been done.
power commander is old school, and intervenes between the ECU and the engine to modify the stock ECU programming.
flashing the ECU rewrites the files in the ECU to do the same thing. No point in both.
tuning is really done with a dynamometer by someone who knows what they’re doing, and could be done to either the ECU or that power commander. Many people presume they can write those files themselves using their butt dyno, with less than ideal results.
long way of saying that if you like the performance you have you could spend some coin and come away with less. Or more.
would the performance change significantly? Impossible to say given your description.
If you want to do it, and want to make it effective, find a performance shop with a dyno and talk it over with them.
if your setup has a power commander installed, that performance shop would likely adjust the files to maximize performance with whatever exhaust system you choose.
That’s even an issue if the state of tune of the engine is unknown. Valve clearances, synchronization of throttle bodies, air filter, spark plugs….
nothing is terribly difficult. It takes time and experience. The results vary on so many factors that it is difficult to predict how it would come out.