Nice butt, Bruno
As far as the WSM capacity, you'd be amazed at what you can fit in there. I've cooked 8 slabs of ribs (four on each grate, using a rib rack, and you may be able to do more if you roll them. Also, there are pictures on the virtual weber bullet site of 6-8lb butts in a WSM. The guy made a teepee out of three butts leaning on each other on each shelf.