I assume you use the i5 because you already have one?
1. Disable cores. 2 cores is enough, dont need the other 2.
2. Undervolt the CPU. Better do this while you still have Windows, since you would want to use stress test tools there. Make sure you raise the final stable voltage by 0.1V before using it in the NAS machine, it will be annoying as hell if it crashed after being a NAS.
3. Use offset voltage rather than manual voltage. This allows the CPU to run on much lower voltages while sitting at idle.