You probably should. That's a lot of rotational weight.

For every pound you add to a wheel, it is similar to adding about 4 pounds to the car (chassis) in terms of acceleration/deceleration performance.

So... assuming a stock weight of 49 pounds... you added about 7 pounds per wheel up front, and about 13 pounds per wheel in back.

That's like adding ~ 160 pounds to the vehicle (chassis) weight. Not to mention effects on suspension (sprung/unsprung weight).

Conversely, many go with 17" rims/tires for track use since it cuts down on weight. A 235/18" tires is about 5 pounds heavier than a similar 235/17" tire. Compared to the stock 18" sedan setup, you could get rims that weigh 16 pounds, and tires that weight about 21 pounds... 37 total. This would be like getting rid of 192 pounds from the total weight of the car ~ 5.5%. OR would be like getting an extra 16HP or so... not to mention quicker braking, better handling, better suspension tracking.