I'm sure Gunner's Leather Mastery should give some attack speed. As a Battle Mage, level 20 green shoulder leather gives +2% attack speed, but in this case, Carbon Fiber would be better since it gives +3%, 1% better at the cost of defense and some additional mastery bonuses (which is probably stuff like critical hit rate, right?).
It's honestly up to you. Mixing and matching is cool and all, but losing some of the armor mastery sucks.
I'm sure that late level armor gives better bonuses than the armor options though. I hope.