Appreciate your observation. I am only cutting very light material. I watch the temp gauge and never does it bog down with what I am cutting. I just do not like the loud noise for hours on end. I set the engine RPM at 1200 or so, make good time and relax.I don't understand why such a big deal on using 540E or even 1000 RPM at less engine speed. For the OP to be running a 6 ft cutter behind a 25 HP tractor, clearly he is not mowing anything thicker or dense at all. If it was putting any sort of the load in the engine, pretty sure the OP would notice it as the engine would bog down quite hard.
I do so much work with 540E, even loading it down really good and had absolutely 0 issues. At 6400 hours, I don't think the OP had any issues either.
I only ask that the engine make enough noise that I cannot hear my phone in my pocket ring.