The leverage doesn't come from the chain, it comes from the shanks, so with the curb rein attached, you still have some leverage from the bit but not the pressure from the curb chain. I used to ride a cob in a mullen mouth pelham without curb chain (he's in a single-jointed snaffle now) because he didn't like or need the chain, just a tad of extra leverage when he got strong. When we were getting him back in a snaffle we tried a mullen mouth first - NO brakes at ALL. He didn't take the blindest bit of notice. So a mullen mouth pelham sans chain is NOT the same as a mullen mouth snaffle.
Pelhams should be straight bars (or with low ports) anyway. Maybe with certain links in, but never a two-piece mouth. Leverage + nutcracker actions equals a very harsh bit, and no horse really needs that.