The idea of 1000 rpm PTO is to run more efficiently. If you have the 1000 rpm geared gearbox on the implement, then use the 1000 rpm PTO, but this is not always something that is available on the implement. Adapting 1000 to 540 is not the best thing to do as you would run the engine at the lower end of the torque and power curves being less efficient overall. Now if the implement is something that doesn't require a lot of power, then okay, but brush mowers especially, need to operate at the speeds they were designed for to do the best job and easiest on the tractor powertrain.
