That Helm, having a yellow socket and +12 stam bonus is a perfect example of when to use a +10hit +15 Stam gem. Or you could even use a +20 hit gem.
Many tanks have mixed views on hit/expertise since the are only 'threat stats'. Personally, where socket bonuses allow I have a lot of hit and expertise hybrid gems to keep me capped.
Remember, IT (for example) is on the spell hit, and when there are less raid buffs out there (eg 10 mans) I've seen it miss twice in a row. Being expertise dodge capped, I've seen abilities parried twice in row.
In my opinion, DK's should care about hit/expertise more than other tanks because of how our rotations work.