That depends on what you are going to keep under them. If you kept the higher-light stuff under the 30 and the lower-light stuff under the 15s, then maybe. A 15 over a 2x2 area at 30" deep is not a lot of light.
This is a really tricky question because it is all about the inhabitants and placement.
The diffusers will cut the output - I would skip them if you just use the 15s even if they will blend the colors better.