I don't know if this helps, but long ago I had a touring bike built up on 26" wheels (back then there was a brief fashion for this sort of thing). After a number of comparative tests with its predecessor (same material and geometry, but 700c), including a few club time trials, I decided it was about 2% slower overall. Acceleration was better, but I think it lacked the flywheel effect that the larger rim diameter gave.
For a tandem, though, I'm not sure that it will make so much difference. I must admit that I've only ridden tandems with 26" wheels, so I haven't got any direct comparison, but the extra weight will compensate somewhat for the reduced angular momentum of the wheels, and with my 8 year old on the back I need every bit of help I can get with acceleration.
For a time trial 2% is important, but just for a fast ride it probably isn't going to make a noticeable difference. I probably wouldn't bother changing the wheels, as it would be a lot of work for little gain. My son is a bit of a speed freak, and I find the best way to satisfy him is to descend one of our local hills.