I don't think it would work well as a grill, you just won't get the temperature and you won't have any direct heat for searing. I do things like chicken thighs on there and then blast them on the kettle when they hit the right internal temperature so that you get lovely juicy chicken with the charred outside.
It's strange that you couldn't get it up to temperature, 200f (93c) is very low. Was that with the top and bottom vents open wide? I tend to find I can get to 150c with the bottom vents pretty much shut due to air leakage. How much fuel did you put in?
If I was cooking a pork shoulder on an unmodified WSM I would stack the ring with lumpwood charcoal and then light a half chimney of lumpwood, when the chimney is ready I would dump the contents onto the unlit charcoal (this is the minion method) and then build up the WSM with the vents all open, I would then fill the water bowl and wait for it to reach about 100c before closing down the bottom vents to half to try and achieve a steady 120c.
Did you have a grill level thermometer in there or were you just using the one in the lid? On the WSM you really need to be monitoring the internal temperature of the food to know when it is done as well as the grill temperature. When a pork shoulder hits 89c internal it's time to take it out, wrap it in foil and a towel and leave it in a picnic cooler for a couple of hours before pulling
