A sway bar is not going to make for a harsher ride driving down the highway, but you definitely can experience some increased ride harshness during cornering with a larger (stiffer) anti-roll bar. The bar connects both sides of the suspension together, and the stiffer you go the more suspension independence you lose; so what's happening to one side of the car is going to happen to the other as well, and you're going to feel it.

I think most of the bars available for the 370 are designed to work with factory components (especially the Stillen kit as I think it's the softest of the ones available) and you shouldn't run into any issues. What you want to do is make sure you don't go TOO stiff as the car will lose its ability to roll and allow the suspension to work properly within it's geometry. Again, with the bars available to us right now this shouldn't be an issue, but there definitely is a line where you go from excellent handling to crap if you cross it.

If I may ask, How can sway bars make the ride less comfortable?