"Guys - what is the likelihood my amp is actually delivering a constant 2500/channel to each channel - at least long enough to really put the drivers in any danger?"
tough to say without measuring the amp. also, there tends to be some rolloff in amps below 20hz vs. stated power in the spec sheet, not always, but often.
"I can shrink the box to protect them if need be, but I'd be giving up some efficiency, right?"
yeah, that is right.
"Also - I believe it was stated that the stated Xmax was likely a conservative number as well. Even at 15-16mm of excursion under the full 1250 watts, is this really an issue?"
while i don't know, my guess is that is well within the safe operating limits of the driver. i've asked dayton if they could provide xmech, but haven't heard back yet.