Ontheairwaves
My point was not to do with the weight (I don't think I mentioned it), rather the fact that the forward doors would no longer be available to passengers. Not such a problem in heavier metal but very restrictive to evacuation in the 737/A320 class of aircraft. That's what would restrict your pax load since you need to be able to get everyone off in 90 seconds (yeah, right !!).
Wino
I wasn't particularly worried about cockpit crew egress. As you say, they have multiple routes whatever the configuration.