Well I am actually still on FCP 6 and still running MacOSX 10.8.5. I have a lot of other expensive legacy software and it became more of a hassle to try and get it working with each new operating system update. AFAIK, FCP 6 and 7 are pretty much the same under the hood, they just added a few features to 7. How did you get it running under El Capitan, let alone Sierra? In the past, I just used the migration assistant when moving to a new machine or operating system, and it worked. Is that what you did?
Recently I tried installing it on El Capitan by just copying the program and support files, but it crashed on startup saying it wasn't compatible. Got the same problem with Mavericks. So I ended up dedicating my Mini to video editing and keeping it on FCP 6 / Mountain Lion. Works very well for me, I am in the middle of making a documentary from 480i footage that I shot in 2002, and have over 200 other legacy DV/HDV tapes that I will be capturing after that. So my present needs are pretty simple.
But, looking ahead, I have the free trial of FCP X installed under El Capitan on an external drive and have been playing with it. As a test, I used the 7toX app to convert my current project and it worked very well (subject to the known limitations). I think I could probably adapt to FCP X fine, but not while I'm in the middle of a big project.

Also playing around with DaVinci Resolve, since it's free.
FCPX seems to run fine on my mini (2012 2.5ghz dual i5 with 16gb RAM) but then I noticed it was only displaying one field of the video. When I checked the option to show both fields, it started dropping frames on my external monitor. I'm using a Blackmagic UltraStudio Mini Monitor which connect via thunderbolt. All the project files and video clips are stored on a USB 3.0 drive that clocks at 180MB/sec with the BlackMagic test. And this same machine has absolutely no problem showing full quality video (both fields) running under FCP 6/Mount Lion.
Have I missed some sort of setting in FCPX? Or is my mini really not powerful enough to work with full quality 480i SD footage under FCPX? If so, guess I will be sticking with FCP6 for the foreseeable future,