Originally Posted by scottfsmith

Not a lot I don't think, unless your pressures are extra low. Note that things can "feel" slower with wider tires since it is a lot easier to handle, less jerky. I use 30s on my Roubaix myself, the Paris-Roubaix winner this year had 30s so they can't all that shabby. 80% of the race is pavement.
If you flip the stem you will get another 20mm or so. I have the stem flipped on my Roubaix and things feel a lot better for me that way.