The purpose of the ARB is to prevent or lesson body roll in corners. If you are driving on a straight highway, the floaty feeling is caused by tyres, wheels, suspension only. Adding a thicker ARB isn't going to do much other than emphasize on bumps on the road.
It's like all the big-guns here have said - start with the most obvious and potentially the cheapest (in the long run) choice - change your tyre size. If you have the budget go change to lighter and maybe slightly larger rims. If I'm not mistaken the GM6 comes with 16" rims - so that's plenty big already. Maybe you can go 205/45/16 tyre size.
If after that it's still floaty - then look at your suspension/spring setup.