It is truly a preference when it comes to the scape. Two islands is a great approach because it allows for a better flow through the tank and minimal dead spots. I would maybe try to rotate the left base rock with the far left point pointing to the left, front corner. That natural cave like area will most likely become a fishes happy place. By angling the rock you will be able to more easily see it while in there. I like the varied heights in the two island formations. If you can, try to make more openings in the left structure while you are stacking them. You get two things from it. First, the fish have more to swim through and around and second, you move away from a solid formation.
The rock on the right structure, the left base rock- it looks like it has a nice natural flat surface that may give you a nice base to support the rest on that side. The other thing is if you rotate it down it may open you tunnel structure you have on that side a little more. So rotate counter clockwise one side (if that makes any sense what so ever).
As far as more rock, I have about as much rock to tank ratio as you and I love it. I don't like the overly stuffed rock tank. Again, personal preference, but any more rock in my actual DT I would hate it. I put more in my sump to make up for the rock.