The dimmer portion of things might be difficult, but not impossible.
For your setup, I think 15 LEDs might be a little light. 24 might be a little better (8x3 grid). It will give better overall coverage with better overlap from LED to LED. Depending on how heavily you plan on planting the tank, very few LEDs will make it harder to get light to areas that are shaded by other plants.
In regards to your optics choice, what made you decide on 25 degree lenses? Something like that will give extremely high PAR numbers that would be the equivalent to over a 250W MH over the tank. For the power levels you are looking at, you can run the setup with a wide angle lense. Something in a 60-80 degree range. That will still get you very high light levels, almost to the point where you will have to dim it down to be more practical. The only reason to use the lenses is to increase perfomance on the substrate. If your tank was shallow (12" or less) you could easily get away without optics.
Color wise, no blue. Not necessary here. You can add a few reds, but they will tend to show up quite badly as red streaks in the tank. You can diffuse them, but you might just be better using a mix of cool white and a few warm white LEDs.
Have you thought about what you are going to use as a heatsink yet?
EDIT: the pictures you have for the tank layout at the bottom of your first post are not working.
Regarding the heatsink, I am checking on options but have not yet decided. Actually my tanks are open top. The lights will hang from light bars (link
to my thread on the DIY cabinet + light bar will give you an idea of how the lights will be held).
As such I have to DIY the light fixture as well which should be light weight, hold all the stuff (bulbs, driver, heatsink etc.). As such I am looking for light weight boxes but have not found any yet. The fixture will probably dictate the heatsink options. I will post the design once I think of it and find the material for it. Need a cover too (thinking plexiglass) but not sure if I need it since I have the optics. Also insulation will be important.
Any ideas on the fixture? Something 24"X15"X4-5"?