Generally speaking, it's not worth unlocking HK-51 on a pre-50 toon, which is a sad oversight on BioWare's part. You have the gearing problems that you mentioned (he comes in level 50 gear, but it's greyed out and has no effect on him until you reach level 50).
Perhaps more importantly, he gains no affection from any conversations in the game, aside from those you have with him on your ship after you've used gifts to increase his affection. He's a completely silent presence as you level, and you're actually missing out on companion affection and character moments if you travel with him in lieu of one of the original companions.
The conversations are not always a bad thing if you are making dark side choice and you only other companion is more light side, like Mako.