zDPS or "Perma-Serenity" there is. Achieved by Gogok, Leroics Crown and max CDR on all available slots. I think one guide said it is like 81.55% CDR with Gogok stacks.
Basically this setup, with a gogok gem, gets you to the highest possible cdr in game if i'm not mistaken. It gives you almost 100% uptime on serenity with fully charged gogok. Obviously 100% uptime on epiphany. The dps from this build is low (obviously, the Z stands for zero) and hybrid builds have a hard time to get to a decent dps output while retaining the high cdr.
CDR cap in general? I don't know. There are diminishing returns so technically, with the gear available, there should be a cap at some point. Not sure what it is though.
Adding 15 Gogok of Swiftness stacks you end up at 81.74%, resulting in 0.35s "overlap", which usually makes up for the lag/latency.
The hard cap for any cooldown reduction is 90% (even with Channeling Pylon you'll never see more than 89.99% in character screen).
Edit: Just realized that the CDR calc is finally updated for 2.1 - http://bannedofgamers.com/index.php?cdr/
You can check my monk to get an idea: http://eu.battle.net/d3/en/profile/Jamoose-2376/hero/19832211
Basically this setup, with a gogok gem, gets you to the highest possible cdr in game if i'm not mistaken. It gives you almost 100% uptime on serenity with fully charged gogok. Obviously 100% uptime on epiphany. The dps from this build is low (obviously, the Z stands for zero) and hybrid builds have a hard time to get to a decent dps output while retaining the high cdr.
CDR cap in general? I don't know. There are diminishing returns so technically, with the gear available, there should be a cap at some point. Not sure what it is though.
Edit: Damn you guys are fast.