This guy picked up 22 mphs.
Go ahead and watch all long drive videos if you dont believe me :)
That was interesting.
I have to say, after watching a few of his videos and seeing basically random passers-by step up and clock themselves on his SSR - that I think the SSR might read a little high.
There's also the issue that some of that increased mph has come from a longer AND lighter driver. The longer shaft probably won't do his ballstriking any good, and the lighter head (even swung faster) won't do his distance any good.
Even so, that looked to me like evidence of very impressive gains - based largely on flexibility work, swinging a shaft as-fast-as-possible, and getting instant feedback on clubhead speed.
If you were hellbent (like, say, the OP) on increasing your clubhead speed, why wouldn't you give this approach a try?