The belts are sided with an anti-submarining bit built in, so it would release and lengthen the shoulder strap on the left hand side to prevent it pulling you to one side. It's not designed to be used with HANS, well your seat isn't. It may or may not put your neck under more load. I shouldn't think it's any different than the effect of a regular seat belt though.
The age old harness/roll cage debate. I just think it's rubbish really, but others seem to have a strong viewpoint on it. The likelihood of the roof caving in and it compressing your neck is a very small one probability wise. I.e., not significant. Your head, your decision at the end of the day.