The damage difference diminishes a little at level 50, with my current gear its 2100 damage with Rapid Shots vs 3200 with Unload. Rapid Shots is still the best to use in most cases as you can stay mobile. Only when you are stuck at range for some reason and have no reason to move away from or closer to your target should you use unload.
Sorry to correct you on this but you should never use TSO with Unload. At long range use it with Death From Above. At close range if you can afford not to move use it with Flamethrower. Both abilities do superior damage to Unload, cost more heat that you save, and have the same 3 second channel time that lets you vent heat.
what part of "this is not about the viability of the skill" do people not understand?