I have to agree with everyone. You must understand no matter how well you feel you have trained your grey, instinct is stronger than your training. Your grey WILL fly away. That I promise you.
As far as wing clipping, I am an advocate of it; all of our 4 birds have theirs clipped, but as they are growing out, yes, they will be clumsy and fly into things if we are not careful. Especially if you have a large picture window.
Please......do not allow your bird on outings if they can fly. I agree with the earlier posting ...it was wonderful...in that their bird goes on all kinds of outings but is CAGED. In fact, there are several companies now that allow you to purchase a backpack of sorts with a huge view window for your bird and with a perch inside as well. Even if your bird`s wings are clipped...when they are growing out, they can fly a little bit and if they get startled, they can fly up in a tree...rest....then fly again to another tree. This happened last 2 years ago with our Jenday Conure. We were ignorant enough to think we could take him outside. Thankfully, we found him. That was the last of his "outside forays" without a cage.
I have never tried the harness.......best of luck with your decision.