I don't get on the page much anymore but was browsing. As you can see from my sig that I own a C5500 35GS Seneca. I only have 54K on it so just broke in. What I did was air bags and Super Springs with Bilsteins. The bags helped but still needed more support in the rear. The Super Springs raised the tail enough to put more weight on the front axel. That alone improved the handling. There is more that can be done to improve load and road handling such as track and sway bars. On the 06 model the rear springs were 15500s which would allow the rear to ride low when road loaded. The bags raised the rear a bit but not enough. The Super Springs gave enough more clearance to move the CG more fwd to improve handling and steering. On those early years 06-07 could have the springs upgraded by Moreryde to 19500s which eliminated the need for Super Springs. Unfortunately that ship has sailed for the 06-07 Seneca.
Best Regards