You probably have enough light to keep the lower to medium light demanding SPS.......If you catch the SPS bug, and want to keep just SPS you will probably want to get into the more finickier brightly colored stuff, you may need to add a couple of bulbs....but you won't really know until you try. SPS not only require the light....but good flow.....and great water.......if your other two are good, then a lack in light won't "kill" them, but you "may" not have the best colors or growth. As Justin has shown there are some killer T5 tanks that blow others with MHs out of the water, but I think these guys have few more watts than you do currently......More watts just increases your odds at success rather than push that limit. Try what you want, and if you experience browning and can't get them to color up by moving to different locations(higher), then you will need to add a couple more bulbs......
I went from 150MH to 250MH for this exact situation a couple of months ago....only I had too low of a wattage halide instead of T5s. The main difference now, I can grow anything in any location in the tank, rather than just up on the top of the rocks under the lights. What it comes down too: having more options and easier to do. For instance, I have been able to buy a lot of brownies and force color back into them with the amount of light I have where it would take twice as long or longer to get them to color up with less wattage. T5s aren't my personal favorite for this application,(SPS), and like the bulb options and overall color and intensity of MH compared to T5...(IMO T5 light looks washed out)......but T5 is a viable option for someone wanting to get into SPS......I don't think however that there are "that" many hardcore full blown all LE T5 tanks around, but perhaps it hasn't been around enough for those peoples tanks to fully grow out........Ill just say, in SPS only tanks, I don't think they are the average type of lighting used......I still think the SE MH bulb is reigning champion there still.....heck, DE bulbs are still a minority also which is what I have......but then again so is 2 part supplementation instead of a Ca reactor also. Time will tell I suppose if T5s will be as common for SPS tanks as MHs have historicaly been.
I have 692 watts over my tank currently....but the wattage number is just a ballpark number, what is the important factor is intensity.......Thats why T5s and MH work for SPS.....focused, intense lighting. PAR numbers are a better reference for discussing quality of lighting produced.
And you know this for fact because....?
And I believe my tank is probably deeper than yours at 20" high. 
. my water is in good shape. my 30g was very stable and that I have moved it over to the 55 it went through a cycle but its ok now. I do have to work on the cal..... I think I will go spend some of my christmas money and get me some coral.....
