I definitely agree with ONE on-use Mastery trinket, but since it's a 1min CD, I didn't find that 2x of them was as useful as having a haste-bank (personally).
I also stayed gemmed for haste, and at 15% exp (mostly because I'm cheap/lazy), but it worked just fine for me/us.
That is all to say that both Butler's and my opinions, while different, will yield the same results. Gear in CM's is all about getting you the first 10% and the last 10%, the middle 80% comes from practice/execution and comp. You can do it with full stam gems or full parry gems, etc. Vengeance is the most important thing, and proper CD management allows you to stack V to obscene levels in CMs. That, of course, comes from practice.
And no, I am not advocating gemming stam or parry, but trying to illustrate that you shouldn't stress a ton over getting your gear
just right, but rather go put in a few reps of the dungeon and you'll have 9/9 in no time