When the heater is for 12V and 24V you won't see a big difference in heating up. You could try to raise the output of your 12V PSU to 14V to reduce heating time.
( If the rest of your electronics can handle 14V too )
I bought a cheap 600W DC-DC-boost converter to increase 24V to 28V because Duet boards are rated up to 25V only. That was a major difference. OTOH you can only use bang-bang heating mode, but my bed temp still is within 0.2°C.