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Wider tire on stock rim?

I have been driving my 2010 C Class Sport 6M for a year and finally wore the stock Michelin MXHX tires down to the wear bars on the rear. It has the stock 17x8.5 wheels on the rear for staggered fitment and the stock tire size is 245/40/17. I would like to get a little more meat back there when I replace the tires. I know that the 245/40 is made for the 8.5 measureed rim width. I want to put a 255/40/17 back there instead which is made for a 9" measured rim width. The overall diameter is almost the same so it won't really interfere with the height or Revolutions Per Mile compared to stock. My main concern is the width. The factory specs for that tire as far as the rim widths are from 8.5"-10" so I'm wondering how much different the car will handle compared to having a measured rim width that is 1/2" less than the tire was optimally designed to have. I'm also changing tire brands since these tires fall way short of my expectations in Michelin for an all season tire. I'm going to replace them with the new Kumho Ecsta 4X which got the best review from TireRack for what I'm looking for. The current stock tires have been scary (sliding around and lights blinking at me on the dash) to say the least and I'm not trying to spend the over $500 clams to replace them given my experience with these tires. Thanks in advance for your input.

I went ahead and ordered the two rear replacements in 255/40/17. They should be fine. I will post some pictures of the differences once I get them mounted later this week. I'm sure I will have more traction with the 255 and the UHPAS tire vs the GTAS in 245. Cheers.

255/40-17 are quite popular on the OE wheels

I can see why. I just got them at the house and they are an inch wider than the stock tires.

After driving on the new Kumho's for a little while I really am impressed with this tire. I'm going to be buying them for a while. Dry is like a Extreme Summer tire and Wet is almost just as good. I can't chirp the tires in between shifts anymore. My car just takes off.