I have worked for Sears Autocenter and been a tech for almost 15 years. Going from a 16" to an 18" is a definate difference in weight (unsprung weight).

I would agree the wheel weight would cause loss of power or the feeling of losing power since you are pusing the car harder than you would have the stock wheels.

And I also thought that someone mentioned if everything was the same except for the cob stage 1 , 2...

 

Huh. Stock 16x6.5s are 22lbs I think. My 17x8s are 15lbs. Looks like my bigger wheels actually sprung that unsprung weight, eh?

 

Yes the wheels may in fact be lighter but he tires are the brunt of the weight, then comes the wheel weights.

Unsprung weight (http://www.musclecarclub.com/library/tech/weight.shtml)

An important concept is "unsprung weight." This is weight that is not supported by the suspension of the car. This usually includes the the weight of the wheels and tires as well as a percentage of the weight of the suspension itself, including control arms, anti-roll bars, shocks, and struts. Reducing unsprung weight is the key to improving handling. The lower the unsprung weight, the less work the shocks and springs have to do to keep the tires in contact with the road over bumpy surfaces. An easy way to reduce unsprung weight and improve traction is to replace stock wheels and tires with special lightweight wheels. Note that as the wheel diameter or width increases, the weight of the overall wheel and tire package increases, thereby increasing unsprung weight.

 

oem 16" wrx wheels are 16.5 pounds