I had eibach pro springs on the stock shocks and struts and it rode fine, but didn't do much other than lowering the car. I upgraded the shocks and struts along with the sway bars to eibach and now essentially have the pro system plus. Its a nice upgrade, although I can't comment on how it would compare to koni yellows. Most people on here say the koni's are better and they are adjustable so that is a plus. If you're really into the adjustability aspect and being able to dial in how you want the car to handle, i would say go Koni's. I'm more of a straight line performance guy and wanted to all my suspension parts to be under one roof so I stayed with eibach and I don't regret it.
2014 Mustang GT 6R80, Procharger Stage 2 D1SC, BBK 85mm TB, Boss IM, Sai Li fuel system. Circle D 3C, 1-7/8" Kooks Long Tubes w/Offroad X Pipe, Lethal Over-Axle Pipes, Corsa Quad Axle-Back, Eibach Pro System Plus, BMR K member, Shaftmasters DS, Forgestar CF-5 Wheels and Nitto NT05 tires. Lund Tuned
Lots of suspension goodies and some other stuff I can't remember.