The lights are great, just a question of how many do you need. A par meter will tell you. Ecotech says a single xr15 will cover 24in so you are probably fine. Now if you had all SPS colonies, maybe adding a couple more would be a good idea.
That's not nearly enough for a 90 gallon. Your best option would be to keep what you have and position each over the left and right sides spacing the spread as evenly as possible. Then you could add some LED light bars on each side of you go that way.
How long is your tank? When lighting SPS it's about overall light coverage, not just about PAR. You need enough PAR, but without the coverage your SPS will suffer and two XR15s isn't nearly enough coverage.