That's strange, I never heard of the fact that heaters were only meant to be used to raise the water temp 10 degrees above room temp. If that's the case, then all discus keepers would have to have a cozy room temp of at least 73 degrees in order to raise temps to sufficient temp to raise discus. But if that's what a rep from the company itself said, then I believe that that's the optimal way to use a heater. Guess I learned something new today.
But from my own experience, I've used heaters to raise the temp 25 degrees above the room temp. This was in a unheated backyard enclosure, so the room temp was dictated by outdoor temps. I'm in the belief that one can raise water temps to a unlimited difference above room temps. Just up the wattage, or add more heaters as necessary. And to play it safe, use a smart temp controller so that you don't risk overheating when the heaters (inevitably) fail.