Mike Curtis has been furrowing his bloggular brow about highlight clipping issues in his clever Final Cut workflows. I can't solve all his problems, but I can show you how to avoid giving your waveform a buzzcut when using Colorista.
Watch the screencast. You'll see me apply Colorista, which clips off the IRE values greater than 100%. I fix this by first applying Brightness & Contrast (Bezier), and reducing Brightness a bit. From there on out it's all fun and games with Colorista.
The part I didn't show was switching on High Precision YUV processing to make sure I don't lose fidelity in the rendering.
(This is the technique that I would use if for some weird reason I was actually doing color work in FCP—otherwise I'd use the slightly more complex method I outline in The Guide, as it makes for a cleaner export)