Yes. When using the Minion method the heat capacity of 3 briskets is more than 1 brisket and it will take longer for the WSM to get to the proper temperature.
No. As long as you have enough coals lit to keep the WSM at the proper temperature, 3 cakes in an oven is the same time as 1 cake in an oven.
All things being equal the time difference should not be that great.