As Kenpau said, leds give a narrow spread compared to T5's. Im soon going to do this very mod myself. Im running a MH/LED combo at the moment and a friend is running T5/LED combo and his color, effect, spread is better IMO.
But he went through a lot of trial and error: First setup was T5, T5, LED, LED, LED T5 T5...it was ok but the shimmer effect and color was localized in the center of the tank. The T5's closest to the LEDs washed out the color of the LEDS some. Second setup went T5, LED, T5, LED, T5, LED, T5...the shimmer effect wasnt as drastic, but the color and shimmer was more evenly spread out throughout the tank and looked so much better.
On both set ups he went did white LEDS and Blue T5's, then changed to white T5s and Blue LEDS. The latter setup was better with the White T5s and Blue Leds. The blue T5s cant make corals pop like Blue leds can. And White Leds cant hit all the NM sprectrums like White T5's can.
On my setup Im going to do close to equal amount of wattage for LED/T5's.
4X54 T5s: 2 X Coral+, 2 X B special. And 4 rows of 5 X 10 watt blue leds running the NM range of 420-470NM.