A tank that is meant to hold aggro on a target from equal level DPS. So if you're a T11 tank, you should be able to hold aggro off of T11 DPS. That's just simple numbers. A tank that is capable of holding threat off of a DPS that does 15k isn't going to hold threat off of a DPS that does 25k. This isn't a one size fits all game. However I would question just how much they outgear you as with the recent threat buff your inability to hold aggro may indicate you have other issues.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.