I can't vouch for the accuracy, but someone over in the tandem forum said that he had determined that the difference between 700X23 and 700X25 was 7 watts per tire. If the same applies to the bump from 25 to 28, then you may see a noticeable speed difference. Of course, you're not moving very fast while changing flats. I prefer that my tires be as flat-resistant as possible.