That's interesting that the BMC comes with 28c tires. That's usually a size used on touring bikes. And they are using expensive Continental GP4000 tires, a very flexible tire, instead of the usual cheaper, stiffer tires that often come with new bikes. The air volume and the tire flex combine to really soak up the bumps in the road.

A few years ago, a BF poster had some good ideas on how to test ride bikes. One important idea was to always fill the tires to the same psi before each test ride. It's harder to do when the tires are different sizes. I would try 105 front, 110 rear on 23c; 95 front, 100 rear on 25c, and maybe 85-90 front and 95 rear on 28c. Add 5 psi to these if you are over 200 lbs.

It's all about the volume of air within the tire. Years ago, even 25c tires were only on the more upright, comfort oriented road bikes. Now, many riders are finding that 25c tires are just as fast as their old 23c tires. The other trend is to wider rims, to increase the air volume on 23c tires to make them ride more like 25c.