Hi OverExposed, greetings from Spain.

I don't think your 16-35mm it's that heavy. In my experience with an 24-105 mm (about the same weight) and the Glidecam HD2000 (without arm), the more you practice the lighter it feels. Maybe it would be wiser keep using your lens for a certain period.

Nevertheless, if you are going to primes, keep in mind that focusing it's very difficult in a steadycam (for me, nearly impossible). As you probably already knows, sometimes you need more DOF than a 35mm can provide.

Finally, recording video with the Glidecam, I didn't see any difference between IS and no IS.