I have a tC with the stock 17" rims. They have the 215/40/17 wheels on them. I wanted to know if I got the 225/40/17 if they would be worth the improvement in cornering. Also I might be lowering the car, would this larger width cause a problem.

don't waste your money on new tires once you drop it with a good set of springs and struts you'll have all the cornering power you need
plus i think those would only be an 1/8 inch or so wider
you wouldn't even notice the difference

better tires are always good. the stock tires (RE92's i think) aren't that great. you'll definitly notice better cornering with some nice sticky tires.