SoloQue, my R/F takes roughly 1hr to 1.5 hr to fully get stable. By stable I mean all the metal is equal. Super heating your firebox will only cause premature burn out of your metal. Slow down and let it come to temp on its own. When the middle upper grate reads 250° about 45 min. into the heating we are putting briskets on, that does mean the rest of the cooker is stable yet but it does mean we start cooking.
If your doors are leaking smoke during the warm up then you are sucking air when it is pulling a full draft.
You can seal the doors with black high temp RTV if your cracks are minor I use this for final sealing.
What I do is spray a light coat of Pam on the cooking chamber where the door makes contact, then apply a bead of RTV to the door strap (the door strap needs to be clean)
then let the door down gently with out pressing letting its weight set on its own. Let the RTV cure then trim off any excess and your door is sealed.
I hope this helps.