I had the same issue, I got one off eBay and it was 5.3.2, but had the other peoples stuff on it still. I went to do a reset and it said that it needed to connect, so I connected it and enabled TubeHax just in case, then I just did the restore and it connected for like one second then continued to wipe everything, I also unplugged my router from the modem just incase, so even if it tried to update it wouldn't be able to get anything.
Long story short it worked and I'm still on 5.3.2. I'm not 100% sure, but I'm pretty positive that it doesn't try to update when restoring.