Here is a video that I shot with my GoPro 720/60p.
All finished at 23.98 inside FCPX. Ben you may have seen this because I posted it elsewhere but I figured it fit the topic of this thread.
vimeo.com/41117635
Now I 1st set up a 23.98 timeline and dropped all the 60p material in there. The motion looked like absolute dog turds so I then conformed all the 60p stuff to 23.98 which gave me fluid slow motion.
All the motion that plays back in real-time is the conformed 60p stuff being sped back up to realtime.
And the “super slow-mo" stuff is the conformed 60p material being slowed down even more via optical flow retiming.
There was no way I was getting that smooth of motion just letting FCPX handle the conversion. It was dropping random frames all over the place. I knew I wanted the clip to ramp in and out of slowmo/realtime and the only way to achieve that was to conform first.
To the OP I know this is not exactly what you are trying to do but its pretty similar and it worked for me. There may be better ways to achieve the conversions as Ben suggested. Keeping sync to audio is whole other can of worms so good look with that…
if you use my method you just need to figure out exactly how much you need to speed up your footage by. I think its a 2x speed increase from the conformed 60p material.