This was my thought from the beginning even before the purchase of the RS 425, I went with the xr15 due to the fact that it was just that "flexible", I was able to space out to my liking. Also, knowing the fact that I may upgrade in the future to a bigger tank I was able to keep or add another xr15. But it was never on the horizon for a 30-33" wide tank, I knew that if an upgrade was to happen it would be longer tank per se'. But geezzz, once I saw that depth of a 33" tank I fell in love of the real estate, It may not seem like alot with the difference of 3" but it is, I was able to compare side by side. Thinking about the cons, it was given that the reach to the back of the tank was gonna be a slight challenge. But regardless, this upgrade will now sit on a 40" tall steel stand with a 24" tall tank. Id would just need a 2 step stool to fully manage the tank, and being 5'11 helps as well. Going back to your theory, Id like to keep lower light corals at the sides and on the sand bed and sps more concentrated in the mid center. What bogged me was that the xr15 only had a coverage of 24" vs the xr30 with a 30" coverage, I definitely over looked that part prior to the purchase. So moving forward, unless I find a smoking deal on three xr30's which probably wont happen till the holidays, Im going to ride these xr15"s and possibly add more. That or ditch the radions and get some orapheks LED panels??
The 30 is just two 15s tied together. Distinct advantages to two 15s over one 30.