Just measured my clearance to the back brace. I have two inches still. I have a Total Control front end with the Shelby drop but I dont think it changes the geometry too much. I think I could have done the 4.5". Micah
255/35/18 rubs a little on the frame rail on full lock but probably could use a spacer, it does not damage the tire only polishes my frame rail. No fender mods
245/50r16 on a 16X7, not 100% sure on the backspacing, but I believe it's 4". rubbed a little on the valance panel but after I adjusted the strut rods while doing an alignment and they clear.
I'd stay under 25.6" for a front tire diameter. Even at that size, you may encounter rubbing on the pinch weld at the rear of the wheel well or even the bottom of the front edge of the wheel well as Bryant has described. I've hammered my pinch weld flat and still have to watch my alignment to ensure I have no problems. As far as rubbing the inner frame rail, I think that may be a result of your altitude. Lower the car some more and it will likely not be an issue.
I run a 235-45-17ona 17x8, 4" BS, the tires will hit the lips at full turn. If you flatten the lip you'll be fine. I can't see you going too much wider than this if its lowered and and not modified. This is on a TCP coilover setup and Bridgestone potenza tires
When you are really pushing the limits on fitment, be aware that different brands of tires will have differing overall real section widths for the same numeric tire size. Some brands run fat like BFGs, some run skinny like Coopers, and there is everything in between. Most brands will have spec charts posting actual sizes. Most of this difference arises from how comfortable a manufacturer is in meeting the load rating that they have to hit for a given size. 245 BFGs and 265 Coopers are only about 1/4" different in section width. If going by numbers only it should be 20 mm ... 3/4".