This was taken from my recent Razorscale kill. On this I tanked sentinels and then Razorscale when we got him to 50%. It's not the best representation of a tank&spank, but it should do for a general value. Specifically it should be nice because according to WWS, I received all of two Essence of Gossamer procs, and no power word: shields for the entire fight. Thus almost all the damage absorbed should be from SD and SD alone.
Here's the relevant part of the WWS:
Because I was facing adds and not the boss for half of the fight, the overall avoidance value is much higher - especially since it was sentinels who have a high miss rate (21.6%) due to their dual-wielding.
For this specific encounter, I absorbed 24.3% of total attacks. This was a total of 112188 damage on 71 hits, which is 1580 damage per hit and right about where we would predict it. This was 15.5% of the total damage I received as physical damage, and of the 100 hits I took 71 were absorbed.
What's interesting is that the uptime is much, much worse than what Rawr would predict it to be. With multiple adds beating on me and with their having such a high avoidance, the uptime should be close to 90%. 71% is pretty low. This is due, in part, to weird overlap issues, and partially due to the bugs associated with SD.
Also interesting is how good it was at absorbing damage. This is one of the better places where SD shines; on this fight I put on extra armor (trinket + neck) and basically 'blocked' 25-30% of all damage from each sentinel. The sentinels don't hit hard except for whirlwinds, and the other adds hit almost trivially. Thus, SD should shine as a mitigation tool, and it did okay. This was, oddly enough, a higher block rate than either warrior both in blocked damage and blocked procs. It was also about half as much that was blocked (in damage and in procs) as the paladin.
The next trick is to not be doing tanking the next Razorscale kill and see how other tanks do at the same damage.
By comparison and to show how bad it can be, let's look at the XT-002 encounter. I was offtanking pummelers on this fight. And they're...not dual-wielding and there aren't as many of them. The encounter I'll be checking out is here:
This one didn't do nearly as well. A total of 147 hits, of which only 54 were absorbed - which is a 36.7% absorption rate and an overall block rate of 23.1%. The big problem here was the atrocious avoidance rate. This, more than anything else, should scare you some; avoidance for druids is going to go way down with the focus on stamina in Ulduar being so important, which means that SD is going to have worse and worse overall uptime. What's interesting is that SD procced a total of 197 times in this fight, yet only 54 were absorbed. That's a LOT of overlapping hits, possibly - or a lot of bugged absorptions.
Finally, we get to our horrible tries on Ignis. On this I was tanking Ignis the entire time, so we should see how well SD does against a single target which does massive physical damage slowly.
On this, the uptime was much better. 25 total hits, of which 21 were absorbed is very good and what we'd expect from something like Rawr. Except...that's not all SD absorption. Part of it is Sacred Shields (I received 8). That explains why the average absorb value was so much higher (1740).
Also, SD procced a total of 89 times on this attempt, yet only 21 hits were absorbed. That's shockingly bad.
Finally, the total damage that was mitigated from total absorption was only 7.5%, and that was after armor. When factoring in my armor (and I almost certainly had inspiration/fortitude up) I was looking at around 69% base mitigation. And BoSanc. So the overall physical damage I was taking before armor/bosanc was close to 1.45 million damage. Based on that, SD absorbed a total of only 2.5% total damage. This means practically even if I had 100% total uptime, SD's absorption would have been roughly equivalent to Blessing of Sanctuary's mitigation. If that.
So the long and short of it is that SD really shouldn't factor much into your gearing decisions. While crit rate in theory helps, the bugginess and overlap factors kill this as a really heavy contribution. AP does so little against a single hit that it's better to stack armor; AP and armor are almost 1:1 in value, and armor's much cheaper to itemize on. Avoidance does help, at least. Still, SD doesn't really make PvP gear less viable. It doesn't make polar gear less viable. It doesn't significantly change the game for heavy-hitters. For light hitters it does pretty well, but the chances that you'll want to gear for small hitters is fairly slim.
I'll try and do more testing of SD, but keep in mind that I don't raid often - and when I do, chances are I'll be DPSing as much as tanking.
On a side note, DPS as a cat is quite good, especially with dual specs. It's still as fun as before, and it's great to compete with rogues for top DPS. And as Karthis stated, it's really awesome to see 33k Ferocious bite crits on XT's heart.