The droprate seems okay to me considering we'll be doing this boss for the next many many months, and they are supposed to be used while levelling up in the next expansion. We killed garrosh every week possible so far and have had 3 of them drop so far.
My raid is currently 0/40. Considering how likely it is to get heirlooms you have no use for or duplicates of ones that do nothing for you additionally, the drop rate is a little extreme. I'm also worried I'm going to win the 1h mace instead of the 2h caster staff, if I ever get one, which will literally do nothing for me or any of my serious alts. The 1h/2h are on both Balance and Resto loot spec (as far as I can tell).
We're probably talking 5% chance or less. It is entirely feasible that many people who kill Garrosh every week (starting with week 1) until the next expansion comes out will never even get a single Heirloom. I can only hope the "lfr-style" safety-net for drops is working in some way for these heirlooms.