Thanks for all of the info and insight guys. I just wanted to follow up. The installer came back out to finally get the stove off of the fire bricks and put the new hearth pad under my stove. I told him about the draft and he did a few things to help fix it. While the stove was out he packed around the flue with some insulation (there really wasn't much room for a block-off as I had hoped.) Then he used some foil tape to stop the flow of air coming from underneath the old Superior box. That helped a bunch. I was then able to pack the box with Roxul. I sprayed the Roxul with stove paint to help lock down the fibers as I read on one of the other forums here. I used some fire proof caulk to seal off the little spaces between the old firebox and the stone hearth. I now have no noticeable drafts. I left the surround partially off for the first fire after insulating so I could check temps and remove it all if there was a problem. The Roxul in the box stayed cool (100F) and the fire seemed to stay hot longer. I also took my IR gun to the chase outside to make sure I didn't raise temps there and there was no difference between that and another outside wall. So I think I should be good. Thanks to all that replied.
