I've been looking for a street suspension for my GTV as well. Here is what I have found from my conversations with the venders:
Alfa Ricambi: Ricambi offers three springs sets, sports, supersports and track. The sports are the lightest sprung but aren't available and may not be available in the future. The supersports lower the car about one inch and are 50% stiffer than stock.
International: Thier spring set is 15% stiffer than stock and lowers the car by slightly less than one inch. They recommend a heavier rear sway bar and KYB shocks although Koni and Bilstiens will work.
Centerline: These springs are 50% stiffer than stock and lower the car one inch. They recommend the stock sway bars and Konis or Bilstiens shocks.
Dave Rugh: Dave's front springs are 30% stiffer and the rears are 50% stiffer. Ride height is slightly lower in front and slight higher in rear (?). Recommends stock sway bars with Bilstiens.
Ward & Deane: Alan's road springs are 50% stiffer than stock and are sold with a larger rear bar. They lower the car one inch. Can use either Koni's or Bilstiens.
Other overseas venders include OKparts in Germany, EB spares in England and Beninca in Australia. From the digest, the EB spares suspension is similar to that of Ward and Dean.
www.alfarestore.com has the OKparts suspension installed.
The Bilstiens are generally stiffer than the Koni's. Being gas pressure, the Bilstiens add a little to the ride height. I haven't decided which way to go, but seeing as my vehicle will mostly be a street car and the roads around here aren't to good, I am leaning toward the International or Dave Rugh pieces.