The Eibachs springs you have fitted are stiffer than the original springs
Very few original shocks have the damping necessary to properly control the stiffer lowering springs. Even if they work reasonably well when first installed, the higher rate springs will cause the factory shocks to wear much faster.
By lowering the car and putting shocks that will control the stiffer springs you have unavoidably bought into a firmer ride. The upside is that the car should have improved cornering power, and sharper handling.
A solution for the front shocks would be to use the Bilstein units from the SRT8
The rear shocks on the SRT8 are Sachs Nivomats - incompatable with Eibach springs.
Check with Bilstein America - to see if they have a listing for the rear of an SRT8, that you can use with lowering springs.
While it is absolutely not permissible to mix types of shocks on the same axle, it is permissible to have different brands of appropriately valved shocks on different axles.
These are the shocks that I would consider (subject to availability for the 300c)- : Bilstein, Koni, Tokico performance, - there are other possible shocks. If you research, and find listings, please post them in thread for the benefit of forum members.
(I owned a high performance Tire/Suspension/Accessory shop)