just wondering if a 255 40 18 will be ok on the stock rear rims

They will fit, but have too small an outer diameter which will throw off the stagger. Go with 255/45/18 instead. I assume we're talking coupe here

Lou

yah but i like the look of the tires being more even in height

The car's electronics and handling dynamics were designed around the stagger. Loose it, and you'll throw the electronics into a fit. JMO.

You'll also leave more room between the tire and the wheel opening - wont look good. But, it's your car.

Lou