This is pretty old news, but in case you hadn't heard, or hadn't seen...here are the explanations.
In case you're one of those people who doesn't care about the "why" of it all, I'll tell you straight up: in my opinion, based upon research and math, they are not BiS; and those people who made me think they were are rude because I put a lot of effort into farming up 391 gloves! Yeah, yeah. It's my own fault for not doing my own research and testing on it, but sometimes I just don't have the time for that and I trust other people who appear to be good at their class or are strong theorycrafters. So that you don't say the same thing of me, I try to post my math and findings, at least to the point that it is visible how I got to my conclusions.
In particular it was thought that 2pc H t12 (typically shoulders/gloves) were part of the BiS list for Sub (and Combat), but more the former than the latter. For instance the EJ Sub writer told me 2pc h t12 was ~2k DPS gain, although he didn't give me any real math or reasoning. I took him for his word and thought he had done his testing of course. His guide doesn't give any math (as I've complained about before) or even state an EP/DPS value on 2pc h t12, which is why I had to ask him in the first place. He happened to be on my server up until a few days ago when the guild he was in broke up.
Originally I questioned using a prior tier set bonus, heroic quality or not, due to the massive amount of agility loss from lesser item level/amount of sockets, especially because Sub has such high EP values on Agility, not to mention the other stat loss from using the lower level item. Like I said I didn't have time to look into it for a while, but when I started playing Sub my curiosity increased.
Example: BROKEN LINKS SADFACE
vs BROKEN LINKS SADFACE
Now this data will look even more skewed due to EJ EP values being updated within the past few days.
The two above items have a 76 agility difference and 101 stat difference so based upon EJ EP values 1 agility = 5.65 EP (newly updated) for Sub that is 429.4 EP for the Agility. Taking the second lowest stat, Crit (Mastery is worse), so as not to inflate the data, Crit is worth 2 EP per point. 101 x2 = 202 EP making the EP difference between the two items 631.4. On average, and this isn't stated anywhere I've found that I can recall, I find EP is a 1:2 DPS ratio; if an item is a 200 EP upgrade, it will equate to roughly a 400 DPS increase. If anyone has better findings or formulated values on this I will be glad to check and change my information. That being said if ONE item is ~1263 DPS difference, you would expect the other to be similar in difference due to the item levels being the same, so you could say the DPS loss from using two 391s for the 2pc over the two 410s would be a ~2500 DPS loss. Assuming the ~2k DPS gain from Herioc t12 2pc is correct (and I have not found it to be) that is still a 500 DPS difference, roughly. Ideally you're not going to use the second worst stat in the 101 stat loss conversion either, so it is really more than a 500 DPS difference. Also from shoulder to shoulder comparison there is a socket loss (but less base Agility difference) so add another 30 Agility loss. 30x5.65=169.5 EP ~= 339 DPS, so being more realistic it's probably a 900+DPS difference using the ~2k value for 2pc Heroic t12.
In my recent findings (based off WoL parses with and without 2pc active, my own and others) I've found the 2pc to gain ~1400-1600 DPS in the best of situations, sometimes far less, and certainly not 2k. This may be due to the fact that everyone in the parses I used are not full BiS t13, but it was a decent test pool. I was careful to use fight durations within a few seconds of one another in length.
db.mmo-champ has been giving me a lot of issues with my java lately and the ability to have roll overs, so if they don't work, my apologies and you now know why. It gives timeout errors frequently.
-Ochiru
I'll fix those links later......it is dark phoenix gloves (391) and blackfang battleweave gloves (410) if you want to wowhead it....I've been quite sick for weeks and I just don't have the mental ability to deal with it right now.