80mm exhaust on our cars are dyno-proven, 20+ at the wheels. Ask Nadine.
It's a turbocharged car, so yes, bigger exhaust pipes will make a drastic improvement.
I would also recommend babying the engine for about 500-1000 miles... you may want to change the oil after the first 100, you'll see that its black as all hell (all the builup and crap from the motor sitting).
Run some Mobil-1 and also use some Marvel's Mystery Oil on it.
Stuff works miracles!
Have fun with your new motor and let us know what happens when you get the new pipes!