I like your thinking! To carry that thought just a little 2mm further the fastest rolling (and most comfortable) wider 700C tire I have found is the 30mm Grand Boise Cerf. Very supple thin casing and tread so make your own judgement as to suitability at your desired pressure. I measured it to be 3 mm tread thick with calipers which is the same as I measured a Pro Race 3.

Volume of 25mm tire is 12.5 *12.5*3.14 = 490.6 volume. To scale up to tandem 30mm tire 15*15*3.14 = 706.5 volume

I actually gave up using it on our tandem because we need a more robust tire but I use it on my single. On good roads this it is a tandem option. Looks may be a problem since it has old school natural color side walls. Note The web site calls it a 32mm tire because that is what they measured it as on wide rims. The tire is labeled 30mm. Pinch flats will be a thing of the past.

http://www.compasscycle.com/tires_gb_700_32.html