I tried that method earlier and it didnt work, along with disable service and various auto start apps as well. As soon as I reset the phone the systemupdateservice would just disable itself again. Even managed to mess up the google play services app to where it would force close non stop.
Oddly enough, I found this app in the play store which I guess runs a script on startup. It kills (or enables?) the systemupdate service and finally lets the phone deep sleep