Not only does the disc have to come off to change the guard but you also have to pull the rear hub/bearing
Best bet is to cut the old guard to remove it, then remove the strengthening rib on the new ones to create an open 'V' to allow it to pass over the rear bearing housing, makes no difference to the strength of the guard and makes it a 10 minute job
I also believe you can use the rear guards from the 2wd V6 Eos as that uses the 310mm rears albeit with a 2wd rear hub so the bolt holes will line up with the GTI, these are open 'V' type from the factory so will slip past the bearing