I've had many cars over the years and spent loads of money improving drivelines to handle power output, upgrading brakes and suspension the list goes on and the bill for just the parts in this regard is huge.
The GTS is a no brainer, all has been done for you already, heavy duty gearbox, tail shaft, diff, half shafts, bigger brakes, MRC, diff cooling, torque vectoring etc
Add to that it is supercharged and legally supercharged you wont get in trouble.
The exhaust is 2.5" standard it works fine and sounds great...no real need to go 0.5" bigger...what for?
Stock form with a tune only they put down 350rwkw on average thats what has been put out now from multiple shops.
Fuel economy goes to **** if you put your foot in it, but i have seen normal driving anywhere from 11.5-13L per 100km
If you are looking at used ones try and get a 2014 build with a build number above 1250 reason being is somewhere before this number they had the spring loaded isolator in the supercharger which needs to be replaced with the solid ones, all builds above 1250 have the solid one.
The auto is faster than the manual and it also drives exactly like a manual car down shifting early itself if you are giving it a spirited drive, it will hold 3rd gear at 100km/h if you back off to 0% throttle, when you hit about 70-80km/h when you stomp on the brakes it will then shift into 2nd and help slow the car down...just like you would in a manual car.
You definitely need to install a catch can from day 1 the blower likes to suck the crank case pressure out.
I've driven an SSV redline and a Calais and a VF R8...none compare to the GTS, it's better because it's way ahead.
In stock form I have raced a few ricers and none have even come close to me, in the quarter mile a mate of mine has run 12.2 at 122mph stock...it is a heavy car off the line but it moves once it is moving.