If there was a big cost savings I would check out the Dayton as well, sure - and I'd also compare excursion capability for linearity and output considerations, among other specs - but otherwise like you show, it's pretty clear the Sundown is likely the better choice.
My thought on "why suggest with the two Ultimax 8's NOW" is because I didn't realize before that they would be THAT squeezed, from a diameter perspective - especially if you are recessing.
Seems like a good, if not great option to maintain (or exceed) output with a smaller, easier diameter.
Curious why didn't you include them in your model? Aren't you shooting for a 2 ohm load? I'd think 500 to a pair would be nearly perfect, and in 2 cu.ft - you'd likely even want it smaller for that 0.7Qtc, which would give you an easier installation overall:
Attachment 10936
And from that you are right, technically you could even fit three, but that spare probably wouldn't leave you with room for a middle on, or you'd be squeezed in the same way as the 10 on the end.
There probably IS more risk with [these] two eights [in particular], to supply enough displacement to break your seal, as opposed to a single ten - but I don't know the excursion or Qes (or BL) on all the above. Note if you modeled up one eight on 500w, two would just be a mental shift of the line 3dB higher... or you could simulate by fudging the power number to push it 3dB higher.
Glad you highlighted the JL in the comparison - you can tell from the .28 Qts that it would likely underperform in a sealed box. That's really surprising to me - people who shop for shallow subs are usually squeezed for space, they really need all the "small box sub" alignment you can give them... and it's easier to design a sub with LESS motor strength than it is with MORE - I'd think if JL simply made the coil longer on this sub here, they'd easily lower Qes, Qts, and increase Xmax all in one fell swoop (provided that didn't make thevVC former strike the back plate - maybe a bump would be needed)
Anyway - just wondering how that option compares. But of what you modeled, especially if the Xmax is similar and the Qts/Fs is more "sealed box friendly", I'd buy the Sundown and be done with it.