Here's my workflow.
Create a new (higher) video track in your sequence. you can control-click on the track panel of the timeline or go to the sequence menu and "insert tracks."
Move the clip you want to adjust the speed to this new track, and lock all other tracks including audio. You can do this by option clicking on the lock icon on the track panel for this track. this locks all other video tracks. Shift F5 will lock all audio tracks. You do this so no other clips are adjusted.
Adjust the speed of the clip by selecting the clip and hitting command-J.
Select the clip in the timeline and control click and choose send to: motion
The clip will open in motion
Hit command-7 to open the timeline
Select the clip in the timeline and go to the inspector
in the properties tab, click the little triangle next to timing to reveal the options
For Frame Blending, change it to "Optical Flow"
Save.
Go back to fcp
Render the clip (make sure it's fully rendered - the thin blue lines at the top of the timeline should be a dark blue, not orange).
Good luck. Post back if anything's not clear or you have any problems.