Welcome to our forum.
A 5.5 kw heater will maintain temperatures better than a 4 kw, all other things being equal.
A 5.5kw will also use more energy and sometimes requires a greater size GFCI.
For example, Beachcomber used to use 4 kw heaters standard. A 750 (with a 4.5, 3.5 bHP and 4 kw) used to require a 50 Amp GFCI. Now with the 5.5 Kw the same tub requires a 60 Amp GFCI. Why did they change? Most likely because after 30-40 minutes in -10 degree celcius the tub would start to lose heat.
How can an element that provides the exact same surface area, heat more?
When BSing with a CAL spa salesman, he mentioned they have models with dual elements a 1.5kw and a 5kw. I believe he said the 1.5 will come on when the spa is only a degree or two below the set point and the big one when the spa is more than that from the set point. Just a gimmick?
As for the Cal Spa guy- If it is as he described, it seems to be a total gimmick in my opinion- Id suspect this may have been a misunderstanding of a convertable system- one that would allow you to have a choice of 30 or 50 amp operation... But the dual element thing, like the dual wet end pump, in my estimation, defies logic.
If it is a warm environment then it might make sense to use only 12 amps to hold temperature? Sort of like a variable speed pump, we could have several steps of heating depending on if we can hold temperature. Only add what we need? It seems that a variable heater would work well with either two speed pumps or a small versus larger pump to maintain heat rate. Saw another heater last night that was rated 1.5/5 KW or something like that. Looked at too many brochures and web pages to remember. Having more choices could be useful feature unless it complicates the control and heater selection and/or makes cost higher? Wonder if this heater question is similar to the pump size issues I see raised on the web. The small circulation pumps are hazardous? Or a good feature? Are there economies to be gained by owner/manufacturer or is using the main pumps advantageous? Why do the smaller pumps get branded as one of the biggest maintenance issues. Does a smaller motor wear out faster? Does a smaller pump wear out faster? Or is there a difference in quality of hardware used?It appears that a different design is pounced on and mauled by the big players here. So if you don't do like we do, your design is no good? Or if it is our idea it is great but if your idea it is a gimmick? Most comments were pretty good. Tell me more about the circ pump debate.