Smarteq. Your car is not right. We drive 80-85 on the freeway everyday and the car handles about the same as our old Honda 600 coupe. In fact the smart feels heaver and will do 90 + with ease.
If I could suggest: don't lead the dealer in solving this problem by offering any solutions (hard not to do for us gear heads.

) But rather look the sevice manger in the eye and say "Fix It".
One of the problems you have is, there are no other Smart owners around that could go for a ride and give you a second opinion. Perhaps you could test drive the dealers demo and get an idea.
Best of luck.
A2Jack.