Shoot the footage undercranked in the first place. You'll never beat doing it in camera. Otherwise I'd leave FCP's native options and go to Twixtor or oh what's the other one, there's another speed altering one out there that is on par with Twixtor I think.
They did it in the camera but it was a music video and they screwed up the frame rate of the play back. The shot at 36 images per second but i have to accelrate it a little to make it in sync with the music. I just got twixtor, what is the best settings to accelerate a clip?