he'd still be pretty good, maybe a slight drop in rebounds or points or maybe not drop at all.
Yao has some good post moves, but lack in athleticism. He does block out pretty well though but can't jump, so probably a drop in block shots.
He already has a good/great jump shot to begin with so at 6'10" it won't really make much of a difference for him on offense. Maybe a bit harder in the post, but everything else he should be fine.
I also would like to point that he might have a slight longer career if he was that tall. I believe I was reading something a few years ago that his frame doesn't really support a 7'6" guy. But then again, all speculation, dude retired and guessing really isn't going to help bring him back to the league