RMS is definitely the wrong measuring stick if you're looking to compare cab capabilities - either published max SPL or displacement are much better indicators.
Ampeg 810 published max SPL: 130dB
Big Twin 2 published max SPL: 133dB
Note: while that's a significant difference, given that's max SPL, that will require the maximum usable wattage being pumped into the cab for it to be realized - that's really where RMS comes into play. The Big Twin, per Barefaced, can get louder than the 810, but as far as I can tell, it'd take double the amp to do it.
Also, as far as the issue of dynamics, it's not possible for me to argue with your completely valid perceptions, but I don't believe that there is a technical reason why increased excursion would necessarily lead to decreased dynamics to the best of my knowledge (which is admittedly limited). There is at least one other thread that I seem to recall reading recently discussing these elements that went into greater detail from an engineering perspective; I'll see if I might be able to dig one up as a reference.
Thank you for correcting me.
As stated, I have very limited engineering knowledge so my terminology might be impaired as you can see.
I am not aware of any physical limitations of excursions or displacement, what I can do is only report my findings by having thoroughly tested all previously mentioned gear.
It might be true that the BT2 is in fact twice as loud as the Ampegs 810 with a more powerful amp. I haven't tested that out so can't say. What I can definitely say is that while using the same amp, the 810 definitely got way thicker and seemed to moved more air (at least sonically to my ears) than the BT2 did. Hard to describe really, best way of describing it would be that the 810 sounded huge as an 810 would while the 2x12 sounded like a 2x12 respectively. Also the 810 had a greater dynamic range or as I would explain, it was smoother when transitioning from quiet to full attack notes. I also had extremely similar results when comparing the BT2 with my now owned Aguilar DB410, previously owned SWR Goliath Sr. 610 and even the relative crappy Hartke 4.5 XL so we might have something here. I have to really emphasize the fact that I am here only speaking on behalf of sonic perception, not relying on technical facts etc. It is worth saying that to the musician sonic perception > technical specification so that definitely plays a big role in this industry.
And between the DB410 and ampeg 810 I can again say, that the 810 sounds like double the DB410 regardless the Watt figures and all that. OTOH a 2x DB410 would certainly smack the ampeg 810 volume wise and I also slightly more prefer Aguilars voicing but that is irrelevant in this case.