Then you should be fine. Just be sure that your COBs really need 12V, then put in a 6 Ohm resistor in series with each one, and those in parallel. Conveniently, 6Ohm seems to be a common value for those 25W resistors. It will dissipate around 1.5W. This drops your efficiency 20 percent points compared to a constant current supply with the same efficiency as your 15w supply.
This is, BTW essentially what those 12V LED strips do, only that their LEDs require around 9V and the rest is dropped at the resistors.