If you're already using 6 iu of GH, the greatest potential benefit is the appetite stimulation. Of course, this benefit only applies to those trying to grow and in particular those who have rapid metabolisms--either genetically or because they've already added a ton of mass and are forced to eat far more than they want to. For these people MK can be a GREAT tool to help them progress.
Yes, adding MK on top of 6 iu GH will cause your GH and IGF-1 levels to climb even higher, so in that regard it could further benefit progress, but keep in mind that going from 6 iu to 10 iu isn't going to make a shit-ton of difference. It will make "some" difference, but nothing dramatic. It would be like adding a few hundred extra mg's of test into the 500 mg you're already using. It will help a little bit, but other things would probably help more at that point.
GH and IGF-1 is no different. If you already have fairly high GH and IGF-1 levels, increasing them another 50% will help, but not as much as some other changes you could probably make.
The good thing about MK-677 is that it is so cheap, so if you're a serious bodybuilder trying to maximize growth (from a GH and IGF-1 standpoint) while watching one's budget, it's good a choice.