I used to run GFO, but didn't ever notice a huge difference, and still got algae at the time. Maybe it reduced it, but wherever the PO4 is coming from was more than the GFO could handle.If you have that amount of algae, you should always be reading 0 PO4. If that is any higher, you have something that leaks it. It does get buffered in rocks etc so maybe thats leaking it but normally that amount of algae is also able to pull almost everything out of buffered stone so you really shouldn't be measuring any PO4.
If that's the reason, you can always add GFO to pull it out.
I don't have much SPS, but do have a montipora that has survived the infestation. It's not growing much, but it's surviving.With that .amount of algae, SPS corals are already gone.