fitted a set of the 820's to my dt for last years september whistler trip,,, all i can say is, wow. they are great. cant offer any comments on the old vs new, as i came from magura louise bats. these were good as trail brakes, but as i progressed as a rider and became faster, i noticed how poor the louise were. theyd fade really bad on the long bike park trails.
the saints put an end to my brake fade issues, i can honestly say i have never noticed any kind of fade, even dragging my rear brake down whiskey jack, or the other steep fire road decents, the brakes dont fade at all. the lever is great, the feel of it, the reach, the adjustability, reach throw, everything about it. so much better than the maguras
as for the finned pads, id say they do make a differance in keeping the temp down, i have used both stock finned pads, and none finned aftermarket pads, (nukeproof cheap ones off crc) i noticed the brakes run hotter without the finned pads, but cant honestly say i noticed a drop in performance. they still work just as well with none finned aftermarket pads.
im running the icetech rotors, not the finned shimano saint ones, as these are only available in centre lock at the mo.
the only issue i have had is the changing bite point. most of the time the pads will bite at the point i have them set to, a few mm from the bar, but sometimes, maybe 1 in 10 times the pad will bite right at the top of the levers pull, then another time it will bite in the middle of the pull, then go back to normal. they were quite inconsistant.
though i have to say i didnt really notice this on my last whistler trip in september this year. so maybe it was a bedding in process. i know alot of the journo's on the press launch complained of this, and there's been a few posts on here about it.
over all though id have to say i am more than happy with them, and now all my bikes run shimano brakes. the podium runs saints, my endo and stylus are on xt. all are very good powerful brakes. you wont be dissapointed.