Everything looks in order as far as gemming and all, if you are taking more dmg than the other tanks then there is a problem somewhere in there with either not keeping HS up, or turning your back to the boss. Myself and our warrior tank take far less dmg than both the feral and DK. As far as threat goes while using SoI, make sure you are pulling with AW and DP to ensure you get off 2 ShoR during the AW. Other than that, threat should not be much of an issue, and if it is you can seal flip throughout the fight to get a threat boost. Ensure that you are popping AW on CD and DP to build more of a threat lead as well. On heroic content and add kiting for nef I solely use SoI and have a spec specific for those two encounters with their own sets of glyphs.
If threat does become an issue then ensure your rogues are ToTing you and hunters MDing to keep the mobs glued to you, some guilds or RL's may give you friction and if thats the case then you may want to look for a new home. For example, a few weeks back I ran all the farm content while being expertise and hit capped from reforging and post a top 10 parse in every fight I was not tanking adds, so when I said I needed it on Halfus because I was tanking our first burn target there was no issues to get them for the first 30 seconds.