FWIW, I don't think phyto, in and of itself, will do anything for SPS. Maybe secondarily, in that phytoplankton feeds zooplankton which in turn is fed upon by SPS. Also apparently phyto can benefit tridacnids, although I never noticed a difference between having dosed DT's in the past versus not dosing DT's. Basically all I ever did by dosing phyto was create a whole lot of nitrates.
You may notice a benefit to your SPS by feeding rotifers though, and of course you'd need a phyto culture to sustain your rotifer culture.
(Whee!)