I love the Fla live rock I bought, lots of nice colors and mine didnt look like cement. You may want to shop around who you buy from. I am sure a lot has to dow ith how many years they have had it un the gulf and what kind of rock they are using to culture from. I think these folks get their rock from mines and it is just ancient dead coral rock.
I would be happy to email you with the place I dealt with though you can just do a search on Tampa and live rock and you will find them. They are the first company to gain a lease from florida for this purpose.
I own some fiji live rock and while I like it , I like the FL stuff better. If you buy it, get the "decorator" or "coral rock" grade. Costs about 5/pound but it will come with brain corals, encrusting sponges, wood sponges and a few hard colors (Golf ball and one other I can't remember the name of) as well.
The cautions about hitch hikers are correct, you will get some mantis and possible bristle crabs. Since you'll most likely need to recure it anyways, just pour some Soda water onto the rocks (outside of your curing container)and watch em shoot right out of it. They hate the stuff.This technique worked great for me and as far as I know I got them all out.
I am not terribly sure what density issue is, but maybe someone can explain that. Your mainly just looking for a place for all the benificial micro sea life to live in and convert your tanks waste products into less harmful by products.
Now all the above being said, I have not bought this rock via mail. I vacation in Tampa a couple of times per year and stop by the store just before I drive home. I live in NC and it takes about 11 hours to get home. They pack my rock in water so much of it survives and little curing is needed. Your results could vary alot if you had it mailed to you.