A single ported (@600W) will have higher output than a dual sealed (@1200W) below ~22Hz, above that point the sealed box will have more output. The ported design will also be flatter, but you can EQ the sealed enclosure to be flat as well.
My guess is that the reason you don't see maple panels much is that most places that would use maple panels make them themselves. If you were going to make your own maple panels you might as well just use solid edging on ply, it probably wouldn't be any more difficult.
I'm going to be building something very similar in the near future. My plan is to simply use a single threaded rod (on each side) to tie the shelves together and then use some L-brackets to anchor it to the wall. That should give it plenty of stability.
They are fairly accurate for anechoic response, but you'll have to factor in "room gain" to get a more accurate idea of how it will perform in your room.
If you drag you mouse over the word SPEAKERS in the upper left hand corner it brings up a pull down menu with each line of drivers where it lists the specs and prices.
If those are the actual prices Adire just priced themselves out of the DIY market. Who the heck is going to buy an 18" Maelstrom with 24mm of Xmax for $500 when you can get an Avalanche 18" with 27mm of Xmax AND XBL^2 for $400? Sorry about the rant, I'd just be very disappointed if this was...