How much of a difference would 215/40 be? I definitely want to go thicker and would like to gain some performance aspects as well if at all possible.
I don't think I understand what you are asking.
215-40 will be best as its closest to stock but I like 215-45 because a) the extra "5" in the sidewall makes the ride so much more pleasant around the shitty roads here and b) 215-40 was f&&king expensive so I opted for the much more common 215-45.
Sorry to interject here, but I thought I would shed some light on this given my experience in the fitment segment of the population. Given the stock 205/40/17's-
205-Width as taken from the widest point measured from sidewall-to-sidewall, measured in mm.
40- Sidewall height as a percentage of width, i.e.- 40% of 205mm = 82mm from wheel to tread, also measured in mm.
17- Size of wheel, measured in inches.
With that, a 215/40 will be 4mm taller in sidewall or 8mm in total diameter from tread-to-tread. A 215/45 would be a total of 14.75 mm taller in sidewall compared to stock, or 29.5mm total diameter. The taller sidewall of either non-OEM tire will place the outer edge of the tire closer to the fender-liner as well as raise the car by that diameter. Visually it will make the car appear lower when viewing the tire/fender relationship while making the car appear taller when viewing the side-skirt/ground relationship. I use
http://www.willtheyfit.com as a general guide when doing fitment calculations. A lot of vehicle handling characteristics can and will be altered by this, which makes it a great tool for tuning a vehicle to fit your needs.
Note-Going with a larger or smaller circumference than OEM will result in your speedometer/odometer being off by a percentage.
On a final note, sizing isn't very regulated within the tire community so you have to pay close attention to the little details if you're going to be pushing the limits of fitment. My 215/40/18 General SS-595's had nearly the same tread width as my 235/40/18 Michelin PS3's because the width of the Generals were taken at or near the tread while the Michelin's were taken somewhere along the sidewall.