Before I bought my GS last summer I was looking at a tripple black '96 LT4. I took it for a drive with the top on, and the later with the top off. I did notice a slight vibration in the car with the top off but nothing too bad.
With the GS, I've noticed it more, but I think it's cause she's equiped with the Z51 suspension and without the support of the roof she just twists a little more at cruising speeds. I've been up to 100+ with the top off and she's perfectly stable, a little more bumpy but stable none the less. Tought it is kind of un-nerving to be running the twisties up here in Oregon with the top down and hear the top pop and creak in it's brackets as the car flexes around the turns.
Jason