Andrew at Polo Reef has a coral lab in his home. He is probably the only one I have ever seen run an actual semi-controlled experiment...
Single system, two tanks, MH vs LED, he observed faster growth and better color in the MH tank.
Starts at 3:30 and he even states "you'll fry the s--t" out of your coral" is you try to PAR match LEDs to MH.
Now, there is lot to be debated here...did flow differ in each tank? Were the LEDs optimally tuned? We don't know...we just know the observation he reported...and he is a big LED lover.
We all have our own opinions and experiences, and some of us have a lot of expertise in lighting (definitely not me!). Take this video for what it's worth, but I tend to trust someone with a 17,000 gallon reef in their home with SPS colonies bigger than my head than a typical hobbyist. At the very least, his single system, two-tank approach is an example of how you generate some actual evidence...more evidence than I have seen anywhere else the last 5 years.
Now, does this video explain the OP issue with slower growth when running LED? No, don't think so. You should be able to have great success with leds. Clearly, coverage can be an issue if you don't have enough units or banks of t5/led strips. Too many other variables that could be contributing though...don't think you can only blame the lighting at this point. You'd have to switch lighting to MH/T5 and observe for 6 months to see if things improved...and even at this point, that would be considered a repeated measure study, without a true control...so your conclusions still wouldn't be that strong that your slow growth was due to the lighting (if things did improve)...just some thoughts. Good luck!