Curb Chain?

Will putting a curb chain on my full cheek twisted snaffle help put some brakes on my horse?

Also...are they legal in the show ring?

Not legal in the showring (i'm fairly certain of that)

I've never actually understood why people place a curb chain on a snaffle bit (no leverage = curb chain decoration (??))

What kickshaw said - on a correctly fitting snaffle a curb chain would have no effect. They're not designed to be used together.

A twisted wire snaffle is quite a harsh bit anyway. If your horse is still running through it it might be time to think about why?