You have to realize. . . the stock BBS rims areforged aluminum. They're already very light to begin with. . . so to find 18" rims that are as light is going to be VERY difficult, and even more costly. . . if even possible.
If you get a set of Prodrives or Volks that are forged, they should be less than that, but they'll still be pretty far off from the weight of the stock 17s.
There are quite a few 18" rims that are substantially lighter than the stock BBS 17's, here are just a few:
SSR Type C(not the SAME as U.S. SSR Competitions, these have proper offsets for fitment on USDM STi Brembos)
18x7.5(5x100 pattern) +48 offset = 14.2 lbs.($415/wheel @ Gruppe-S.com)
18x8.5(5x100 pattern) +48 offset = 15.3 lbs.($429/wheel @ Gruppe-S.com)
**wheel weight specs converted straight from SSR Japan website
Here they are on World One Performance's STi in 18x7.5 size:
18x7.5(5x100 pattern) +48 offset = 15.99 lbs.($600/wheel @ Gruppe-S.com)
**listed weight is a confirmed figure from Garrett at World One Performance
**in order for the STi to fit these wheels, World One Performance had to modify the hubs and brake caliper brakets
According to 5Zigen, the 18's tip the scale at 17lbs. per wheel. However, I cannot confirm any fitment on the STi regarding these rims.
Those rims all sound light but what's the weight of stock BBS rims? Plus i'm thinking 17 tyres would also be lighter than 18 tyres. So i think either way you'd be adding weight to the car with bigger wheels. I'm not an expert though, somebody correct me if i'm wrong.
Hattori-Hanzo is exactly correct; total wheel/tire combo weigh is really what matters. For any giiven wheel/tire combo the smaller diameter will always be lighter: less unsprung weight and rotational mass. There probably are 18" wheel/tire combos that are lighter than the stock 17" combo, but the same 18" aftermarket combo in 17" size would (most likely) be lighter still. If ulitmate performance is what at you're after, find the lightest 17" combo. The 18s sure look way cool, though!
I put BBs 18 inch RE's on my 04 STI. They look great but I race with the 17 inch stockers. I can actually feel the difference in acceleration. The car jolts forward with 17's but not as hard with 18's. I also have street tires on 18's and R compounds on 17's but I still feeldifference. You should only race with 17's. 18 for the street. :-?
JimD is correct. If you are looking for pure performance go with 17's. If you like the bling of larger rims, get 18's and be happy (I must admit it would be easier to clean that dam brake dust off with 18's) . Some people, however have had troubles with SSC 18's and warping rims. If you don't mind 17's and light is what you are looking for, check out the thread I started regarding custom fabricated magnesium 17's projected to weigh around 6.6 lbs from the MiM group in italy. http://www.imprezawrxsti.com/postnuke/index.php?name=PNphpBB2&file=viewtopic&t=13888