Any time you purchase suspension lowering parts such as lowering springs or coilovers, you are in effect reducing the suspension travel.
Eibach makes Prokit springs for the SC300 and SC400 that will drop the ride height about 1 inch all the way around. These are the only springs that function within manufacturer's alignment specifications. The 1-inch drop is, in my opinion, what the stock ride height should be. If you want a sportier, modified look, you should lower the car a bit more. However all springs that lower over 1 inch will operate slightly outside of manufacturer's camber specifications (by one degree negative camber or less), which creates a negligible inside tire wear. This gives you use of about 90 - 95% of the tire's total lifespan.
Espelir makes springs that drop the ride height 1.5 inches in the front and rear. Intrax makes springs that drop the ride height 1.5 inches in the front and 1.3 inches in the rear. Tanabe makes various springs that drop the ride height about 2-2.5 inches all the way around. Eibach Prokits designed for the JZA80/MKIV Toyota Supra are also compatible with the SC300 and SC400. The Eibach Supra NA Prokits drop the ride height 1.5 inches in the front and 1-inch in the rear. The Eibach Supra TT Prokits drop the ride height 1.3 inches in the front and 1 inch in the rear. OEM Supra NA and Supra TT springs lower the SC about 0.5 inch but require Supra strut shocks.
Any OEM and aftermarket suspension designed for the MKIV Toyota Supra will also work with the SC coupe. This includes all Supra coilover packages. Ony thing you must consider when shopping for Supra lowering spring packages: you must swap the entire strut-spring assembly from the Supra rear end, not just the springs. Meaning if you purchase Eibach Supra Prokits, you must also purchase Supra-compatible strut shocks, because the rear strut perch on the Supra sets lower than the rear strut perch on the SC. If you attempt a Lexus shock with a Supra spring, you will actually lift the rear end of the car higher than stock. The front struts are the same on both cars however. You have the option of buying the OEM Supra shocks (cheapest), or aftermarket Tokico, KYB, etc shocks. It is recommended that you replace all four shocks together for uniformity, instead of just the rears if you go this route. Supra NA and TT shocks will fit the SC300 and SC400 fine. You do not have to worry about these compatibility issues when you buy Supra coilovers.
-rich