Most tire manufacturers list a 275/40R19 requiring a minimum 9 inch wheel, so while it will work on an 8.5, no one that really knows what they are doing is going to recommend it. I see a ton of warranty claims come through that could have been avoided if the retailer had just matched the proper tire to the car/driver. Many things can go wrong out on the road, why add to the chance that something will?
Really the widest you should go on an 8.5 rim is a 255/40R19. Go with a better (stickier?) tire for your needs, don't mask a cheaper tire by going wider in the wrong size for your wheel. A Michelin Supersport in a 255/40R19 will kick the crap out of the stock 245/45 Pzero Nero product for ultimate grip as it's not compromising grip for all-season performance.
This info is on any tire manufacturers website for those interested.
Edit - Always make sure your load and speed rating are correct when changes tires/sizes!
Your resident "tire geek".
2012 Mustang GT, Lava Red Metallic, Brembo, 401A, Cashmere leather, pedestal spoiler and hoodscoop. Mods - Boss shift knob, Weathertechs, GT fender badges, Steeda Ultra-lites/Tokico's D-specs/Steeda HD mounts & phb/GT500 iso's, 20% tint, Steeda UCA, Roush Axlebacks, Bassani X-pipe, no reso's, Painted lower valances, Boss mudflaps, MMD hood struts, MGW shifter, 275/40R19 Michelin Super Sports all around, Whiteline Sways/LCA's/Brackets, Steeda CAI, Bama 93 tune.