When you upgrade to 18" wheels, you will also be running lower profile tires. While there may be a slight weight penalty for the larger rims and tires (ie heavier rims and tires) you may not really notice it in a seat of the pants scale. Also, what would have greater impact on acceleration is the overall diameter of the wheels.

For example - using my "fuzzy math" logic

A 255/40-17 would have effectively the same diameter as a 265/35-18

Tire = 255mm (width) x.40 (the aspect ratio) =102mm where 102 is the height of 1 sidewall

Rim = 17" = 17x 25.4 (where 25.4mm~ 1") = 431.8mm

So.. 2x102=204mm (2 sidewalls - one on the top and one on the bottom of the rim) 204mm + 431.8mm = 635.8mm

Tire = 265 (width) x.35 (aspect ratio) =92.75mm where 92.75mm is the height of one sidewall

Rim = 18" = 18x25.4mm = 457.2mm

So.. 92.75x2 = 185.5mm 185.5mm+ 457.2mm = 642.7mm

So, the difference between a 255/40-17 and a 265/35-18 is about 1% - which may not be something you notice.