"Do you hit a point of diminishing returns after a certain enclosure volume?"
kind of, but there are additional considerations that complicate things.
smaller enclosures are less sensitive, so require more power to hit the same spl on the bottom end. as the driver moves in-stroke, it compresses the air. that compression pushes back against the driver. the smaller the enclosure, the faster the pressure builds against the driver, so the less the driver moves. the less the driver moves, the less sound you get. however, once your driver runs out of excursion, you are pretty much done.
a total of 1400 watts will push 4 of your drivers to xmax in 6 cubic feet total, whereas it will only take 1200 watts to do the same with 8 cubic feet total. so, anything in the 1.5-2.0 cubic foot per driver range will work just fine for that driver and a 1000-2000w amp. i don't know how much that driver has past xmax, so if you go with the more powerful amp, go with the smaller enclosure. just to be double clear, if you go with an amp with 2000 real watts such as the behringer nu3000dsp, go with 1.5 cubic feet or even a little less cabinet per driver, for a total of 6 cubic for four drivers. if you put two drivers in each of two cabs, that would be 3 cubic feet per cab. sorry for being redundant.
how do you plan to build the curved backed cabs?