Yep, back then T-5's were not well known and what few that could be found were expensive. There was no such thing as T-5 HO (also called HE).
I wasn't referring to the ballast heat, but to the tube heat. I would think the extra wattage would increase the heat and probably/possibly shorten the life of the tubes. That's nothing more than a guess since I have never increased ballast wattage to any of my fluorescent fixtures.