Ive got the wsm stacker and I love it. there is enough room that I made a 2nd grate mod to it. Did 12 racks of huge loin backs on my wsm with zero crowding. I actually perfer to us my ws with the stacker even when I only need 2 grates as its more manuverable for me to remove the stacker to get to the top grate than it is for me to remove the top grate of the wsm t0o get to the lower grate. if that makes any sense. feels like Ive got 2 top grates now.
very little effeciency issues and oddly if anything my wsm is more consistant from the top to bottom with the extra room. you do have to consider the additional meat acting as a heat sink though. much different cooking 3 racks versus 12