Yes, 250º is 250º but does it stay at 250º?
When you put cold meat into a hot WSM, the meat will begin absorbing heat energy, reducing the ambient temp, until everything comes back up to temp.
The same thing happens in your kitchen oven. The big difference is the oven's thermostat will detect the temp drop and kick on the heating elements to compensate.
Unfortunately, unless you're running a BBQ Guru type device, your WSM has no way to "kick on" more heat from the coals.
Cooking time, once up to temp, really doesn't take much longer. It's the temp recovery that adds more time.
Recall the rule of thumb - every time you lift the WSM's lid, it adds 15 minutes to the cook time.
But ... how much longer are we talking about? When I do 1 or 2 butts, the overall time really isn't much different. All sorts of variables are involved from cook to cook that adding an extra butt or a couple more racks still falls within a ±25% 'tolerance'.