There are all 3 PTO speeds on my tractor: 540, 540E (750) and 1000.
Most used speed on my tractor is 540E, because 540 rpm is achieved at 1660 rpm of engine (nominal engine speed is 2350 rpm). This is most convenient speed for mowers, rakes etc.
Ordinary 540 (achieved at 2080 rpm of engine)is just for tiller or power harrows.
Speed 1000 rpm (achieved at 2300 rpm of engine), we use occasionally, only for run circular saw or something like this. It is bought for use in future (who knows?).
Using 1000 for purpose of 540 is not recommendable (dangerous) because of several reasons:
1. Engine runs on too low speed (1240 rpm on my tractor) - weaker lubricating, weaker cooling, easy to overload engine.
2. Turbo doesn't "work" yet - bad combusting, black smoke, coal buildups on pistons.
3. Low engine speed is on rising part of torque curve (decreasing torque when load decrease engine speed)- very unstable for variable load, easy to stuck engine you "hit" something.
4. Hard to use hand throttle, because of engine speed "floating" (not so important).
5. EASY to overrun PTO attachment - VERY DANGEROUS!!! for your /forums/images/graemlins/shocked.gif, or yours tractor life /forums/images/graemlins/frown.gif , as somebody mentioned before.
6. Some more...but I don't know now... /forums/images/graemlins/grin.gif
Just a remind: on some tractors to engage 1000 rpm, you need to replace 6-splines PTO shaft with 21-splines, other can have shiftable 540/(540E)/1000 with 6 splines, like mine has.
My advice: if you can find 540 and 540E (750), this will be more convinient than 1000.
Almost standard setup on new tractors in Europe is 540/540E now, 1000 usually on demand.
Hope this helps,
ZJ