05 GT Mustang,Auto, Dark Mineral silver.NOS,Added stripes and Hood scoop,Borla stingers,Steeda pulleys,SCT Tuner,AEM Induction,Sequential tailights,410s,Kooks LT,Crane cam stg II.
The Eibach Pro-kit does not have a very stiff spring rating. Eibach has always generally been pretty comfortable as far as ride quality, even on the 79-04 Mustangs.
If you just swap the springs out, without changing shocks, you may have a rougher ride than stock, nothing that will break your teeth though... with shocks, the ride will obviously be smoother and better dampened.