I slice the ball so I need a shaft is more stiff because I lag the club open. I swing too hard!

All things being equal, meaning a slice move is not part of the equation, the grip is good the weight shift is good ETC then the following can be said.

In most cases a golfer with a normal tempo, meaning not ultra aggressive in the swing, this is a false philosophy. You are meeting the ball with the club face open because the golfer is not strong enough to "flex" the club closed. You can look at it in this way. The shaft is so stiff (for your swing) that the club stays open and cannot shut down. In order to experience this effect a more flexible shaft is needed.
Now, if the swing is ultra aggressive, Like a long drive guy swing, then consider a shaft that is more stiff. Ever get a senior golfer that says " I am old and need a more flexible shaft" OK you say then watch that same golfer take an abbreviated fast back swing and finishes with the cub pointed at the target. A lot of force is generated in this swing and a shaft that is more stiff will work better.