The stock rears are actually 275/40/18, not 275/35. There would not be any noticeable performance increase unless the new wheels were extremely light, like magnesium. The sidewalls on the 18/19 setup would be slightly shorter than the 17/18 combo, so there may be a very slight handling benefit. If it still has the o.e. run-flat Goodyears, any non-runflat that you put on it will improve the handling, even in o.e. sizes. The run-flats are horrible handling tires. It looks like you have made excellent sizing choices. The rears would only be 1/8 taller overall than the stock rears and the fronts would be almost identical. Not enough to effect the abs or wheel speed sensors. C5's look great with the 18/19 combo. I'd do it for appearance more than anything.
