^^ it really depends on how light your wheels are.
as a rule of thumb, 18" wheels are heavier than 17" wheels are heavier than 16" wheels and so on. this is quite obvious.
however, some companies specialize in making very light wheels that still have big diameters (up to 20"). one such company is centerline..who advertises their 20" wheels to weigh 23 lbs...or about the same weight (give a pound or two) of your stock 16" gts alloys.
diameter does have a little bit to do with it..but most importantly it's weight. if you buy an 18" rim that weighs about 22-23 lbs then you won't really see a Performance decrease. obviously the problem is just finding an 18" wheel that weighs that..the more specialized you get the less options are available.