Heavier wheels will reduce peformance, but not nearly as much as your having a set of 18"s out back. Personally, I don't see your average person as anywhere near likely to tell the difference between two wheels identical in all ways, say 3 lb's difference between them. While it is certainly prefferable to have the least weight possible for a faster time down the 1320, unless you are 100% dedicated to improved times, get what looks good on the street for most of your driving.
If you have the funds, get a set of Saleens for the street and a set of 15" Welds for when you go to the strip. This will VASTLY improve your times/grip and be a win/win!