now theres a good idea i didnt even think about...laminating paper

What i was gonna do is just pick my color of paint and get a thin acrylic sheet (like 8 bucks at lowes) and just paint the acrylic and put it on the back...yours sounds cheaper tho lol.
I too like the branchy stuff a lot. I like having the main chunk of rock, and then having a few good long branches going to the sides/forward/diagonally, and then i like putting zoas or polyps or even gsp on the tips of the branches...i did that once and the zoas were covering the whole branch (which was about a foot long) within maybe 4 months. Unfortunately i traded it up, tho i now wish i'd never have done that cuz i loved that branch.
But yes, you're right...more porous rock is what you want. If you get the really dense stuff that hirocks used to sell before they went out of business, you're not getting as much surface area like you say. So really, it might take say...oh idk...50 lbs of dense rock to equal the same amount as maybe 30 (those numbers are NOT the right numbers...just using them as an example).
Also, if you get dense rock, you're gonna need more of it just to make it look like theres actualy that much. I personally like the look of lots of rock built up high, not just a really low rock formation. If you get porous rock, its gonna be lighter and it will take less to get the rock to "fill in the tank" more than dense...see what i mean?
But lookin good tho

Maybe get some more rock on the left side to fill it in more.