I've got a set of 275/40/18x9.5;s that are going on my rears. They currently rub & don't fit.

I'm going to be having to the fenders rolled soon so that they fit, would I have much trouble if I dropped the car ~1'' or so on springs afterwards?

 

The wheel offset will be a huge factor. With 18X9.5 +45MM you can fit a 275/35/18 with no rub and no fender roll all the way around even when lowered. The 40 series tire will be taller which is why you are having a problem with fitment. Also remember that your final gear ratio will be taller meaning you will be going faster then your speedometer says and the car might feel like it accelerates slower.

In regard to lowering the car, it should not really matter, except you will need to increase the static negative camber.

There is an aggressive wheel fitment thread you should check out.