80% isn't really a "rule" at all - each rating (bronze, silver, gold etc) has 'peak' efficiency, usually around the 50-80% range.
https://en.wikipedia.org/wiki/80_Plus
Generally, a quality "1200W" unit can provide the full 1200W, assuming they have the appropriate Amps on the 12V rail (100A for the full 1200W) as the vast majority of modern components utilize the 12V.
At 100% load, efficiency is reduced slightly. Bronze drops from 85% to 82%, Titanium drops from 94% to 90%.
All that means is (using Titanium as an example), your 1200W unit pulls 1333W from the wall (+11%) vs 638W from the wall for a 600W load if running at "peak" 50% efficiency.
Running at max load, generally, you'll increase heat. Depending on the quality of the PSU, the level of heat etc, you may end up shortening it's lifespan some.
Also, PSUs degrade over time, even quality units - so a 1200W PSU for a 1200W rig (assuming it'll run 1200W most of the time) won't necessarily be stable forever.
If you *truly* have a 1200W rig that'll be running near 100% load most of the time, I'd aim for something like a 1600W PSU for that added headroom (and slightly improved efficiency) .