I too have a mixer with 24v phantom. It's a Fostex mixer, new in 93-94. I also have a 1U rackmount Rolls mixer that is 12v phantom. I have some Carvin SDC mics for overheads that worked excellent on 24V, never tried them with the 12v. Back when I was demoing songs in the mid 90s, I was using an AT4033 into that 24v Fostex board. It worked, however, I didn't think that I was getting the dynamic range that I should. My band rented a vocal preamp unit with 48v phantom. The sound was MUCH better. That being said, I'm not sure if it sounded better because I had the full 48V or if it was an improvement over the Fostex pres.
On another note, I was doing live sound once while recording the show to ADATs. Someone suggested that I use their new SDC mics for overheads instead of my trusted Carvins. Over the 100ft snake and 20-30 cables the phantom just didn't work. I thought it should because my Mackie has 48v phantom, but after thinking about it after the show, I believe my mics ran through the Rolls units, into the ADATs, and then into my Mackie. If that was indeed the case then I was only giving out 12v. I suppose it's possible that his mics 'required' 48v, but most little overheads I've used work fine on 12v. Heck, some will even run off a AA battery.