The g37 wheels are 19x8.5 F 19x9 R

 

i wouldnt go to a dealer spouting off about how you feel the bridgestones are inferior to the kumhos. They'll probably laugh you out of the dealership. Like redlude said, they probably went with that specific tire due to the added weight of the g37 and the specific load ratings of the bridgestones that nissan likes to use.

Performance wise, the bridgestones probably out perform your kumhos on most aspects except cold weather handling and maybe road noise.

I like redludes advice, step up the tire sizes to a 245/40/19 for the front (basically the same as a 225/45) with a 275/35/19 for the rears.

Also I dont suggest you asking for them to deduct $800 for the tires alone. They wont make you any bargains there, just try to get the best deal you can get for the car in total, use the OEM tires and save your money for some nicer wheels and tires...that's what I would do.