Seeing the latency dropped to around 450 ms I gave tanking five mans a try. To make certain I wouldn't be too much of a liability I hit two of the original Cata heroics.
To give you a reference point, I'm used to around 60 ms latency for actual game data, and as low as 35 ms for chat and other low-intensity WoW-utilities. That would be the world and home data on your interface.
I ran quite a few of the original five mans earlier to test out a threat set of mine, one that hasn't particularily changed since then.
The result was a decent overall 10 - 12k dps no matter which instance I ran. A bit higher if there were numerous groups of mobs, which shouldn't surprise anyone given how the paladin tank does AoE.
While two runs doesn't constitute a statistical sample, a drop to around 8k overall dps seems significant. Obviously I was aware high latency would have a negative impact on my dps (not to mention overall tanking performance), but the rather drastic difference still came as a surprise.
Some of it can be accounted for by a greater damage intake. Ie, I'm slower to move out of The Green Pool of Doom. Hence I'm likely to hit WoG more often than before, and that's one SotR less. Still doesn't account for somewhere between 2 and 4k loss of dps.
So most of that dps loss has to have latency as the culprit, and as a protection paladin I'm not even dependent on a crapton of procs for my attacks. I'm bashing most my abilities in according to a GCD based schedule, and thus I should pretty much be able to keep my rotation intact.
If I lose that much to a mere 0.4 seconds latency overhead, then I don't even want to have nightmares about what it'll do to someone who relies on procs.