Large electric boiler is about 3 times slower than small one but uses 3 times more energy. Large RTG boiler with 300 pellet heat is about 5 times slower than small RTG boiler with 40 pellet heat.

Doesn't the RTG firebox and electric firebox already fulfill these roles? The electric firebox is already more inefficient than a solid/fluid firebox, whereas the rtg firebox seems mostly for automation sake due to the relatively low TU output.

The electric boiler is for getting small amounts of hot fluid. When you want to go industrial then do that and get the big Boiler.