I use a mid 2010 27" imac with Ubuntu and everytime (thus far) it has been a real pain to install due to the complicated limbo state of the graphics card support. With lots of 'nomodeset', rescue disk, editing x11 conf etc etc.
fglrx in this case is NOT a good idea, it simply does not work properly with 3d acceleration, neither does fglrx-legacy.
However, the most important thing to do, is make sure you are using the right installer. Use the 'mac alternative'. This (14.04.01) is the FIRST time, I have been able to do the whole installation using the GUI installer and be landed in an OS with properly working drivers with 3d acceleration using a 4xxx card (Mobility Radeon HD 4850 in my case).
So thanks ubuntu ! took a while but great.
Installer I used was : ubuntu-14.04.1-desktop-amd64+mac.iso
From UK mirror
http://www.mirrorservice.org/sites/cdimage.ubuntu.com/cdimage/releases/14.04.1/release/
The actual driver packaged with this is the Community driver as described here.
https://help.ubuntu.com/community/RadeonDriver
Hope that helps some one ( probably me next time I do it! ).
On the note of EFI and booting :
I use rEFIt, I have not had to change it for 4 years, I just reinstall over my previous linux partition. Your Linux partions should not be EFI, they should be Ext4 or something else, heres a print out of mine in case it helps.
bob@bob-iMac:~$ sudo parted /dev/sda print
Model: ATA ST31000528ASQ (scsi)
Disk /dev/sda: 1000GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 20.5kB 210MB 210MB fat32 EFI System Partition boot
2 210MB 100GB 100GB hfsx MAC
3 100GB 300GB 200GB ext4
5 300GB 308GB 8000MB linux-swap(v1) swap
6 308GB 1000GB 692GB ext4 DATA
sda3 here is my main installation eg /
nomodeset
the drivers don't support 3D acceleration.