I have my general solution for the problem. Recently I have installed Ubuntu 16.04LTS Gnome on my iMac with Sierra Mac OS X. The reason was that OS became sluggish and I wanted something faster (and after 17th years with Linux the solution was obvious). I follower a nice tutorial Ubuntu on iMac
Everything went great until rebooting the iMac after Ubuntu installation. The only booting system was Linux, moreover I could not see the GRUB menu... Hopefully it was booting) The Linux works great, just some Wireless and graphic drivers were missing (used Xorg instead), but after plugging it via Ethernet cable to my modem I made all updates and installed the drivers. A link to Wifi drivers installation:
Ubuntu mac wifi drivers
Graphic drivers are selectable via Sytem settings GUI
So, coming back to dual booting Linux and Mac OS X. I played around with GRUB, none of the methods worked for me!!! Pressing "Option key" did not work any more (GRUB has overwritten the iMacs MBR).
I almost gave up to make it before weekend, but then I tried rEFInd:
sudo apt-add-repository ppa:rodsmith/refind
sudo apt-get update
sudo apt-get install refind
was everything I needed to get my dual boot working perfect for Linux and Mac OS X. Now I have at the boot rEFInd booting menu and easily select between booting partitions.
The only problem remained is still with GRUB splash menu, I cannot see any options I guess due to some wrong graphic menu. I tried console mode, I does not work, some error message about graphical regime at the boot. I will report here ASAP I solve it!
Then i did some tweaks useful for all Ubuntu installations:
How to speedup Ubuntu
Especially preload worked for me as a magic: usually sluggish on start Gimp now loads in a half-second!