At that level of power I don't know that it matters.If your box is worth a crap you should be at a point that SPL is well into the ridiculous range either way, and the most possible difference you'd have would be
The stress on your wallet would be higher, and the the stress on your electrical system would be higher.
To get ~5000w FROM your electrical system it would have to be exceptional (and expensive), not that 3.5kw would allow for anything much smaller.
IMO once you reach a level of ridiculous I just don't see the point in further ridiculousness. ESPECIALLY considering SQ is even on the radar here, and somehow it sounds like it's more important to you than SPL -- which I find hard to believe.
As I'm typing this I keep SMH at the mention of SQ at this level of power. It's like choosing which muscle car to buy based on fuel economy.
The box size generalization is overstated IMO. Box size is more related to mechanical damage, and it would take a pretty severe mistake with enclosure design to damage those subs mechanically at anything in the vicinity of 1750w, and 2500's in that vicinity. Too much power will burn up a 3" voice coil though, regardless of box size.