I not sure if you can change the speed of your clips FCP like the speed change in this clip - and make it look that great.

It looks like Twixtor - what do you guys think?

Wait a few seconds for it to load . . .

http://www.filecabi.net/view.php?file=1.mov

Looks more like shot on 35 in slo-mo (48-60fps), then speed ramps were done in the edit with something less sophisticated than Twixtor.

If something is already shot at high-speed and transferred @24, then doing speed ramps is easy in the edit because doing ramps at that point is just dropping frames that already exist, rather than trying to generate frames from thin air (which is what Twixtor does).

But maybe to answer your question, Twixtor could be used for something like this, given you have plenty of time to tweak it out and wait for crazy renders.