I still think you're going to need spacers up front with a +40 offset, even with the Dr. Diff 13" rotors.
I know that compared to my set up, with 275/35/18's on 18x9's with 35mm offset, Dr. Diff 13" rotors and a 3mm spacer, the 255's and 40mm offset show as being .1" further off the suspension, but that's not the only thing to consider. The 255/40/18's are over .25" taller than my 275's, and that will come into play. My tires still touch the frame at full lock, and tire height plays a bigger factor there than offset. Granted, my wheels just barely touch, but I think with a taller 255/40/18 you'll end up with interference at full lock. My guess would be that you'll need at least a 5mm spacer to make them clear in the front.
Not that a 5mm spacer is a deal breaker, but it might be necessary. Heck, I need to go to a 5 or 6mm spacer to finally get mine completely clear if I can manage the room to the outside.
