Love your rock scape... very awesome! Just my $.02, but I see you may already be leaning towards a few SPS. I tried to have one XR30W over my 36x21x24 and it wasn't enough due to shadowing and my desire for a mixed reef with an emphasis on SPS. I now have two XR30W's over my reef and I'm pretty darn happy. Try two XR15W's and you find it's not enough, buy a third.
What are your stocking plans for the tank?? It's difficult to say for sure without that as a guide... basic LPS and zoas just on the rock work and yep two xr15's would give you enough light over the rock but would leave shadowing in the middle... if you add SPS as long as it's at the top of each rock section again in that zone you would be fine... But as corals move and grow and with SPS branch out you will start to struggle...
If you are going to keep a LPS tank consider a third xr15 for the middle. If you have ANY plans for SPS you will need either the xr30's or at least 4 xr15's if they would fit I would go with 5 xr 15's for a SPS tank.