Perhaps not for all auto sound sub drivers, but pretty much for all auto sound subs, which rely on cabin gain for response below 60Hz or so, and since they have that cabin gain they can work in very small enclosures. Those drivers might provide adequate output in a larger room where cabin gain is a non factor, as with the OPs room, but that means a much larger box than what his drivers are currently in.
I am going through the same thing with car subs here:
I saw 2 used subs like mine 100 each, probably talk them down quite a bit...
If it were me, I'd eat the losses and go with a different driver. Staying in the cheap, I'd go with the Infinity Reference 1260W/1262W. I picked up a pair last year on Amazon for $50 or so each. They are pretty comparable to the 12" Infinity Kappas. Even if you have to build a new box, you'd still be ahead assuming they don't budge on the price. Without looking at the specs on the 10" Kappas you may even be getting better performance down low by going to a pair of 12" drivers. Another option for $200 is to go with a single 15"/18" driver like Dayton or SI. You still need to by MDF to build a new box though. To me spend money on 10" drivers for HT is a bad investment unless there is a specific requirement for them.
I think that really depends on how the OP feels about building a completely different box, right after he just finished building this one... The pair of RS265HO-44's will perform very well with this setup
With 1000 watts and 30hz filter. That equals what most 15" woofers would output with 1000 watts.