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.