Henry,
This heater unit is still going strong after 3 years, all 8 heater still function and do so on a regular basis. I have them stepped at 1 degree intervals so first one set at 74 then 73,72,71,70,69 etc. I have a kilowatt meter mounted to the unit so I can make certain all units are operating correctly.
If all 2000w are running I can get 5 degree an hour temp rise on roughly 600g water. So, without taking a lot of complex variables into play you could add another 300w and probably be good.
My situation requires RAPID heating in a worst case scenario (sump out side, power goes out in winter=250g of freezing water waiting to go back to the main display). So my plumbing is more complex than it would be in a normal setting. I can describe it if you really need to know but it involves relays, actuated ball valves and temp sensors in multiple locations, etc. I have had 2 "worst case scenario's" since the tank has been setup and the fail safes performed as expected.
I run approx 1200gph through this heater unit. Again, a guess for you, about half this should work.
I like 2", gives more dwell time inside the unit and the cost/space difference is negligible.
The water is always at the same level due to gravity, not water flow. Input, output and tee's are all at same level so the internal water level is always constant regardless of 2000gph or 0gph.
HTH
Dan