Cannot say I have ever seen a sickle bar set up to run on 1000 rpm pto. If you take one for 540 and run it at 1000 you will simply shake it to death and break something.
What are you really trying to cut? If you are looking to cut hay, then there is not comparison. A disc cutter or discbine is by far the superior cutting equipment. Each has it's little querks to learn. Sickles take lots less power, generally cost less, but run much slower. Discbines with either rollers or flails take lots of power. 100 horse or better is best, but some machines will run well with as little as 75-80 depending on terrain. The only limitations to how fast a discbine can cut are the guts of the operator and the smoothness of the field. I have cut hay with my discbine at 10 mph just to see if it would work, and boy did it ever do the job. Field was too rough to do for long though, normal cutting speed is 6-7 mph.