I believe that the starting product, before trimming, must be 5 lbs.
But as they say in the song.... I LIKE BIGG BUTTS AND I CANNOT LIE!! why would ya want to start with a little one? Anyway..
my interpertation of the rule is that it must be cooked whole, the entire time. If you seperate a piece at 145, the rest of the butt cannot be returned to the pit and used in the contest. If you want to use parts of different cuts, cooked to different temps, and put them in the same box, then cook seperate butts.
I have discussed (with reps) that some cooks will significantly cut around and seperate the money muscle, but leave it attached to the rest of the butt by a small piece. I was even given a demonstration(by the rep/pitmaster) of how this is done... Technically, its legal. however, using the technique above, if you seperate the money muscle and cook it so it APPEARS to be cooked seperately, there is a risk that this would be caught by an experienced judge or judge/pitmaster and questioned. This happened at a contest and the cook had to explain himself. You are risking a DQ for cooking a tenderloin, or parts, or seperating. After seeing this done and the result, I would not take the risk. Additionally, IMO, its splitting the hairs and not the how the rule was intended.
Most of the time, in contests, I use boneless butts and cook at least 2. I rub the hell out of it inside and out and then tie it up. I cook each to different temps and slice one and pull/chunk the other. But they remain intact throught the cook. Once Cooked to the proper temps, you can seperate the parts later. But once seperated, they cant be returned to the pit seperatly.
a few times(not in a contest) I have cooked them openfaced/butterflyied to allow more surface area and bark formation.. and the money muscle cooks open faced. the one(for slicing)I pull at a lower temp, but it was cooked intact just splayed wide open. The result was a much faster cook, and alot of pieces with bark. I personally like the result better when its tied up.
My nickles worth.. Your milage may vary.