Now to the logic of my first point. I suggest you let the idea sink in a bit. Took me something like 20 wipes on the boss aswell. Lets say the leg has an even 6mil Hp, you have 5 dps and every dps is doing 50k (100k on the leg). Now if all of them target the leg, it'll die in 12sec. After that everyone switches their target to the boss and does 18sec 50k dps until the next leg spawns (when everyone has so switch targets and move again). All together this makes up for 10,5mil HP (12sec*100k*5)+(18sec*50k*5). Now the scenario i suggest with just 2 dps on the legs. Those two need 30sec to kill the leg (30sec*100k*2=6mill). During that time the remaining 3 dps will do 4,5mill damage (30sec*50k*3). So in both scenarios the damage done in those 30sec is the same. The difference between the two scenarios is just the number of target switches your raid has to do. No.1: 10 switches (switch to leg->switch to boss)*5 dps, No.2: 2 switches (switch to leg)*2 dps.
Hope I could clear things up a bit for you.
You're neglecting the fact that some DPS, like combat rogues and frost DKs and arms warriors can do cleave damage, which copies damage from the legs to the body, effectively making every GCD on the leg double the damage of every GCD on the body. This cleave was nerfed a bit recently (the body was receiving the doubled amount of damage, not normal damage) but it still makes the GCDs of certain classes 1.5x more effective on the legs than on the body.