Did you turn off Enhanced LTE before the update? If so, the update would have turned it back on, updates to to re enable settings you changed, and that could be causing the high cell standby. Some people (myself included) have seen cell standby disappear or drop to something more manageable (like 2% to 5%) once enhanced LTE is turned off. All enhanced LTE controls is VoLTE with other AT&T phone, which isn't anywhere close to widespread yet.
It's also definitely not fixed in 5.1.1. My Moto X also suffers from the cell standby bug and it's as close to stock as you can get without a Nexus phone. It's not a bad though so Touchwiz and whatever AT&T does seems to make it a bit worse. It drove me crazy when I first got my S6.