The temp drop was exactly what I expected after putting 34 pounds of cold meat into the cooker. It's not real cold today, maybe upper 40's when I started but it is windy. Wind will suck the heat right out of a WSM and make it hard to maintain temps, but there's ways to deal with that. By cracking the lid, you will allow more air to the fire and therefore it will burn more vigorously. It responded nicely w/o having to add more lit coals. Because of this effect, I try to minimize the time the lid is off when using lump charcoal because the temp can really spike on you.
One other thing, did you make sure the lump was packed tightly together? It doesn't matter with briquettes because they are of uniform shape but lump is a whole 'nuther story. You have to fit it together tightly or the fire will not propagate through the pile the way you want. I love lump but I can do without the part of arranging it by hand.
Backyard All-Stars BBQ Team
"Everyone has a plan until they get punched in the face." Mike Tyson