I think extra holes will affect humidity faster than they will affect heat. This is because the melamine of the rack and even the plastic of the tub will hold the heat, as well as the air. Humidity, on the other hand, is only in the air, unless you have condensation, which you shouldn't have anyway.

In 2 of the racks I built, I have back heat. In that case, I added a few holes in the back of the tubs, to help the warm air get into the tubs. This seemed to cause the humidity to go down slightly, I believe because it encouraged the air to flow a little more. That might be worth a try.

Also, one of the easiest ways to make a big impact on humidity is to change the size of the source of humidity, the water bowl. If you use water bowls with a smaller surface area, you'll get less humidity.