Well, theoretically it's 8% weaker. I had a discussion with a guy from Trickstuff and he calculated it. Dunno if 8% is a lot or not.
So if the 820 has 2mm less material, doesn't it make it more prone to brake than the 810?
p.s.: My 1:1 comparisons are made between the 640 and the 820. They have both the same caliper, just with different labels.
p.p.s.: I ran the Zee caliper with 810levers. It's a super powerful brake but as the caliper broke, I kinda lost confidence in it and got a Trialzone instead, which feels equally powerful but, as Ali C. stated correctly, I don't do huge stuff so I shouldn't be in a discussion about brakes' performance.