I had an R2000 w/ CAAD7 frame, probably same era. At the time the R2000 was Ultegra (9 speed 6500) and the R3000 was Dura Ace. If yours is CAAD7, what I'm writing will probably help. Also, I believe the CAAD8 had exactly the same dimensions, but I could be wrong on that.

The CAAD7 frame would take 25mm tires, but 28s were iffy. "Small" ones would fit. The critical area was in front of the rear tire, it didn't have enough clearance with the seat tube, especially if you got even a tiny amount of mud or grit clinging to the tire. The clearance at the stays was OK (just) as I remember it.

If you have a 28, give it a try, but if purchasing, probably stick with 25.