Not that I don't agree with your basic premise that there's little harm in trying any Feature Update if you've made a full system image update before doing so, but the speed of the restore process is directly dependent on both how fast your backup drive is and how fast your system drive is in addition to how large the backup is.
The machine I'm typing from has a 2TB HDD, of which about 1.2 TB is still free, and I use a USB 3.0 backup HDD. It takes about 5 to 6 hours to restore.
Still not a big deal, as if I have to restore I just kick the restore off before going to bed. If I had an SSD in the computer this would shave of a huge amount of time, but I still doubt I'd get down to something under an hour.
There are still an awful lot of systems out there that retain the HDD OS drive they came with. Doing a restore from backup on those is a significantly slower process than when SSDs are involved.