Possibly this is coming from lack of experience...but pure frame dimensions often tell me zip about how a bike rides. I hate to use a tired saying, but riding a bike really is the only way to tell how it feels. Data sheets, sales brochures, manufacturer terms (OCLV, "10r carbon", etc)...all of those really do not tell that tale properly.

As for running a 28mm tire on a Roubaix...I'm not sure on that one. The Roubaix I spent last season on (2009 105) barely accepted the 25mm tires with the brakes all the way open. You may or may not be able to squeeze a 28mm in there...I'm not sure it will clear the brakes though (unless you get a disc brake model).

I guess I have to ask this...are you planning on doing much curb hopping? Perhaps I'm making an assumption as to where you live but in the central Illinois area, I've yet to have to pop a curb on my road bike and that's after 1000's of miles over the past year. Personally, I stay away from sidewalks and curbs, sidewalk to road transitions is something I just avoid.