My 1200 ranges reliably and quickly out to 850-900 yards or so; beyond that it will do it but it's trickier.
To ME, that would be the reason to upgrade, if there even is a reason- not so much to be able to range to 1200 yards, but rather to be able to reliably range to 600, 700, 800 yards. If your LRF800 is already doing that for you then I don't see a reason to upgrade myself.