The amount lower with 15" wheels instead of 18" wheels depends on the sidewall height of the tires you install. If you install 205/40-15 compared to the stock 205/40-18 your car will be about 1.5" lower (half the wheel diameter difference because the tire sidewalls are the same height for the 205/40 tires). However, if you go with a larger sidewall tire with the 15" wheel, such as a 215/45-15, you will drop about 0.88" (7/8").
I'm just wearing out a set of star specs. I do autox and DE. They've been great. But, on very hot Texas summer days they can get a bit greasy on a long DE day. Going to RS-3 next because they seem to like the heat better. Both are great tires.