Time to get Nitpicky!

They fell short on their first attempt by 50WP, clocking out at 750.

They were short 8 ponies.

Without those ponies Rainbow Dash figured they would have had a tough time making the 800. Prior to this, however, Rainbow dash was confident they could reach the 100 mark. Just how much work did he expect those 8 ponies to do?
If you are only taking into account the 50WP they were short, those 8 needed to average a 6.25WP. But the sick one is the one that clocked in 8 the first attempt. Lets say then that he improved to 10, the number Rainbow said they all needed to be at to break the record. Since, again, he clocked in at 8, and even fluttershy managed to improve her speed by 2 points without even putting the same amount of training in as the others, I think it's a safe assumtion he would have been up to 10. That means the other 7 were at an average of 6WMP.

If we take into account the full 150 to get to the full 1000, they would have to be averaging 18WMP between the 8 of them. It would require every last one of them to have achieved a higher WP then equestria's fastest Flier, Rainbow Dash who clocked in at 16.

Rainbow Dash may be headstrong, but this is far to unrealistic an expectation for her to have had. That and I doubt she'd expect that many ponies to have match her speed, none the less surpass it. The numbers make virtually no sense in context of whats going on.

However, I did figure something pretty funny. Rainbow stated if all of the Pegasi, (Since it seemed all Pegasus were to contribute to the Hurricane), needed to reach a WP of 10 or greater, doing so would allow them to break the record. Ignoring the sillyness of expecting some of these guys to double their speed in such a short time (Since the first pony to test their WP clocked in 5) we can figure out the approximate number of grown Pegasus in Cloudsdale/Ponyville. First off, we subtract Rainbow dash's speed of 16, giving us 984. At 10WP a pony, we have 98.4, or rather 99 ponies other then Rainbow Dash. So including Rainbow back in, we have approximately 100 Pegasus living in the area, not counting smaller ones.