Is being a little over GVWR no worse than doing 60 in a 55?

Doing 60 in a 55 isn't right either. It is a speed limit. Lots of people treat it as a speed suggestion or an advisory, but in reality it's declaration of the max speed permitted on that roadway. It's no different than the rating on the truck. It's not really a suggestion, but a limit that the manufacturer has determined to be acceptable. Lots of people like to say that it's lawyers who come up with the numbers, but no one really knows the true motivation behind the numbers. Could be liability, mechanical, or even safety limits. No you probably aren't going to get fined and you truck probably won't immediately fall apart, but when things go wrong then you're SOL. I'm not saying that I wouldn't do it, but I definitely wouldn't want to do it often. If you're buying a new trailer it sounds like you will be doing it often. 300-500 lbs over is pushing it in a 1500.