Don't forget he's also changing tunings while playing. Every time he hits the headstock, he's changing the pitch of a string.
I'm not sure about shredding though. Then again, I haven't listened to everything he's put out. The closest out of the stuff I've heard to shredding is his work with Attention Deficit though.http://www.youtube.com/watch?v=7EaNLt9knfY
Tapping-wise, one of the videos I posted had him tapping on three basses at once. But he tends to do Tony Levin Chapman stick-style tapping if anything.
The headstock adjustment while playing stuff really didn't impress me much. IIRC he wasn't playing very difficult parts, comparatively speaking, while he was doing that.
This Attention Deficit track is cool but I'm not sure I'd call it shredding. It's not Sheehan that's for sure.
The three bass tapping thing was pretty neat but I'm not sure we should count something like that as more impressive than the truly wicked tapping of Levin and Sheehan, etc.
I'm really not sold on Manring jumping Sheehan or Wooten. But no way he should drop out of the top 6 based on what I know. My impression is in the top 6 or so, everyone's pretty close in skill.