Joe, you won't be happy switching from a C4 Cab to a CLK55.
The E55 feels less sporty (regarding the suspension) but much more powerful (no wonder ) compared to the CLK55.
I liked the CLK55 very much because it feels lighter, steering-in in curves is much better, it feels more directly attached to the street and I'm sure that a good coilover kit could do wonders on the CLK55. Add the AMG limited slip differential to the CLK55 (yes, even the CLK55 has traction problems ) and you have a very nice car.
But may I ask a stupid question? Why would you want to trade a C4 Cab for a CLK55? If you need more room for the family, including some impressive luggage space in the rear trunk, the E55 is the car to go with. Otherwise you should stick with your C4 Cab or get a 911 Coupe instead.
Also: ever thought about the Cayenne Turbo?