I could be WAY wrong here, isn't Table B 310.11 more a NFPA requirement than actual NEC code requirement? Was it ever actually adopted into code? (You can blast me because we're still using the 2002 NEC.) Moreoever, doesn't it only apply to feeders #8 or larger?
I can understand the issue here would be heat. However, heat would be an issue in a gutted out panel too? For that matter, heat is an issue in a panel in and of itself. GFCI and especially AFCI breakers generate LOTS of heat... no mention of that anywhere, and in my opinion THAT IS an issue. I always keep Raytec ST-60 infrared thermometer handy and I never noticed any real heat source from a gutter, J-box, service disconnect, or the meter. But point that thing at a panel loaded with GFCI and AFCI breakers and the results are pretty scary to me. On panels with a main, the main is ice cold, but the breakers are almost hot to the touch. Clamp your meter on figuring the breaker is almost at it's trip point, then you're shocked (no pun) it's only drawing 2A.
Heat would also be an issue with large gang boxes and dimmers. Something this section doesn't appear to address either. Of course, heat problems by upsizing wiring end to end. Then good luck with trying to trim out #10 or #8 for a 20A circuit.
I believe gutter fill is far more important of an issue, as addressed in NEC 376.22, but even that section has some oddball provisons.
By the way, heat and derating would also appear to be an issue in some of the panels we so proudly display here, with the conductors all neatly wire-tied together... something I too am guilty of.
From a practical standpoint, here in NYC gutters (and large J-boxes) are used all the time for service upgrades (when new panels need to be moved for space considerations, etc.) and I don't think I've ever seen any of them get red tagged.
Just my 2ยข of venting...
Joe
