hi Crister. From what I am seeing these are accent lighting, with less than 1W per LED
each. They won't do much for you in the way of growth, as the PAR and penetration is very likely extremely low.
Unfortunately the foot print of most high power LEDs won't allow you to add it in addition to the fixture you are currently using.
However, I have 1 of these fixtures: TrueLumen Pro LED StripLights | Current-USA
on my planted tank. It is pretty powerful for the price point, and the form factor would allow you to add it to an existing setup that has T5s on it. The Marine Fusion version might work very well for you.
lighting being a newer technology, it is generally not very cheap. You could think about trying to build your own fixture if you have some basic electrical and soldering skills. Rapidled.com has some powerful LEDs and drivers for a good price.