Hi and welcome to AO!
Congrats on the car, the GT is the best looking current Alfa IMO (well, barring the 8c
) and you have the best engine.
As far as your questions, I have Ragazzon centre and tail-pipes on my GTV, they were on the car when I bought it, so can't comment on any performance improvement, however, the sound is incredible and IMO well worth the cost in itself! It's not too loud whilst cruising or in traffic, but I guarantee that you will find yourself dropping the window and hanging onto gears far longer than necessary, even when just pottering around, just listening to the V6 and grinning like a simpleton.
Not sure about whether you need to get the ECU re-mapped, or if the exhaust needs to be done first. AFAIK, re-mapping involves a new set of operating parameters (fuelling/ignition maps) being flashed into the car's ECU, usually via a laptop, or by fitting a new 'chip' directly into the ECU.
Again, AFAIK, the new maps are specific to make, model and engine type, but once the car is run and driven, they learn how to get the best from the actual engine they are controlling. They do this by monitoring the information received from the gas/temp/speed sensors, and adjusting fuelling and ignition patterns to find the optimum values achievable. So, if I'm right, it would be feasble to fit the new exhaust after the re-map, but it may be necessary to dis-connect the battery long enough for the ECU to forget the optimum values that it had learned, then reconnect battery, start up and the learning process will begin again, taking into account the new exhaust characteristics, and optimising to suit.
NB. The last paragraph maybe complete rubbish, and should not be relied upon as accurate.
The information is simply my interpretation of stuff I have read, and stuff people have told me.
I started this reply at a fairly reasonable hour, then fell asleep on the settee, woke up and forced myself to finish it. Hopefully, you will receive more info to answer your questions, once the forum wakes up tomorrow.