It seems audibly louder. With the surface area of the 810 I would assume it would be on par with that. But its louder then the 810 with just swapping heads at same settings. I have to turn a amp down to be where it was volume wise in the other cabs.
+1 I have a pair of older Mesa Road Ready Diesel 215s. Aside from being a pain to move (I like the Ampeg kickback), they are my perfect big cabs.
For years, I used a pair of Ampeg 810s. There were a few situations (festivals, etc), where I had a 215 as a backline cab and they were always crazy loud. At some point, I go the itch to try 215s. I tried to track down @JoeVictim's Yamaha 215s and he suggested checking out the Mesas.
I could use one cab and keep up with two 120 watt full stacks (though I still use both). With the 810s, I my Sunn 300T was almost dimed, with the Mesas I run it at about 11'o'clock. I find they have a more extensive low end and a more pronounced mid punch.
I really like the sound of Ampeg 810s, they are just inefficient for the size.