You'll need to make a power budget.
I have found quite often that the consumed current is more or less constant for electronic components such as microcontrollers and sensors.
The reason why the voltages were higher before is that the operating voltages of the transistors were higher.
Consumption is largely determined by the clock rate in a microprocessor/microcontroller which is due to the charging/discharging of the internal (stray) capacitors, mostly on the clock, but also for any other changing signal.
The power required to charge and discharge a capacitor is \$C.V^2\$ (the energy stored in the capacitor is half of that, the other half is lost when charging it).
You can see that the energy in the capacitor is proportional to the square of the power. So changing from 3.3V to 2.5V saves close to 40% of dynamic power.
If your DC-DC has an efficiency of 80%, you could save 20% of this dynamic power.
Some microcontrollers have an internal linear regulator and/or a DC/DC. The linear regulator makes sure that the core always runs at the same voltage, which is another reason why consomption depends more on the current than on voltage.
If the microcontroller has a DC/DC, adding an external one is likely inefficient.
Static power computes differently. Approximately it is \$V*I\$, so dropping from 3.3V to 2.5V saves about 25% of power.
But you need to take into account static power losses. How much does the DC/DC consume when there is no load for instance?
So you need to make a power budget:
- How much power do you need (consider frequency profiles, etc);
- What is your static and dynamic power (dynamic power depends on frequency);
- What is the efficieny of the DC/DC (look into static losses);
- Consider this for the different voltages (when the current consumption changes with voltage).
Also, do not forget that you can quite often modify the operating frequency of your microcontroller, which can be a great way to save power. Measure how you are consuming power by putting a "small" resistor in series on your regulator output and by monitoring that voltage on an oscilloscope. Adjust the resistor's value so that it has a small power drop that does not stop your circuit from working, but which enough for measuring it without too much noise (I've used \$10\Omega\$ resistors for that).
Do you need it/what do you need?:
There is of course also the cost of the DC/DC, the cost of designing it in, the extra point of failure, the space it occupies, etc. It's a tradeoff of all of that and the need for a longer battery life with regards to the expected impact on sales and profits.
I went through the "trouble" of selecting an SDCard, optimizing the processor's frequency and SDCard operations in order to increase battery life to a week in an application that was measuring 24h/day with LEDs and recording those measurements on the 4GB SDCard. I went from about half a day of autonomy to 1 week by optimising the program and selecting and understanding the best SDCard. But it was a requirement!