How big are the butts Steve? I have done 4 6lb+ butts without much problem (20hr cook). The only concern I would have would be airflow with that much meat. As long as they aren't 9-10lbers you may be okay. BTW I did NOT rotate the butts from the top to the bottom and they were all done at the same time. I also used water. Hope that helps!
I just finished the third rack mod this morning and it looks real good. Im going to try to do 6 butts this saturday. I will let you know how it all turns out.
Steve, Without doing any mods to the WSM you should be able to fit 3 butts on each grate. Typically in the 6-7lb range...perhaps you might have to fit them in right but I have done it many times before!!