So what does that tell you!
"I suspect efficiencies will vary between different MPPT controllers".
They do - but not by very much. Regardless of controller model or brand, the same "higher voltage = lower efficiency" pattern remains.
If you want, I could post other examples here, just ask.
"Higher voltage means....... significant improvements in cable efficiency’s."
Transmission losses over any given cable run can be as high or as low as you like. You get to decide what percentage is acceptable. An inadequately cabled Serial system could easily be less efficient than an adequately cabled Parallel system.
"The voltage headroom (of Serial connection) gives the MPPT controller more to work with as voltages drop on cloudy days."
But what has the controller been able to do with this extra voltage "headroom"? Nothing! Look at the left side of the graph. Even on very cloudy days, low voltage parallel connected systems are still plugging away at 90% or more while high voltage serial connected systems have plummeted off the chart.
Voltage "headroom" is clearly something to be avoided!