You don't need to go any longer then a 164 unless your searching for a back country stick.
And if you keep pretty much any current board waxed properly, it's going to go as fast as you can handle.
Never Summer uses a very good base material as well. I can assure you speed will not be something lacked.
And the old days of getting a board that's length is between your chin and nose is long gone. Weight and type of riding you wish to accomplish with the stick are the important factors when figuring length. And these new techs with reverse camber ride roughly 3cm longer then they actually are, based on old standards.
I currently have a 161w as my "do it all" stick and I don't plan on going any longer then that. I weigh around 185-200
"When I do good, I feel good; when I do bad, I feel bad. Thatís my religion." - Abraham Lincoln
Moral of the story: If your going to rob people, wear a diaper.