In my experience, any obstruction to the seal on the lid lid (in your case, your temp probe wire) really changes the combustion rate (and therefore the cooker temp). I use this fact to my advantage when I want a hotter cook, like for chicken. I will sometimes place a piece of folded aluminum foil between the barrel and the lid to create a small gap, similar to the gap that your probe wire created. It doesn't take much of a gap in the lid to crank up the cooking temp.
The PBC seems to be designed to cook at around 250-280 with the lid completely sealed and both rebars in place, assuming the coals have got a good start.
I suspect that your lower than expected temps on your first cook might be due to your lighting method. When I've done the chimney method, I pour the chimney coals onto the basket of unlit coals, and I let the cooker sit with the lid open for another five minutes before hanging the meat to allow some of the unlit coals to start greying over. This seems to produce that 250-280 temp range I mentioned above. When I've tried to hang meat right after dumping the chimney, I've noticed cooler cooking temps.
Finally, if you experience lower than expected cooking temps, crack the lid for just a few minutes. It usually doesn't take much to get the coals going again.
The fun part for me with my PBC has been learning to dial in my desired temp for the kind of cook I have in mind....