Spacemunky, you need to get out more. Ride a Roubaix. There's a significant difference in stability, vibration reduction, and sound. In fact, the first thing I noticed when I first rode my Roubaix is the amazing silence of the bike. The two bikes are different in obvious ways.

Thanks for the comments, everyone. My thoughts were to go with 25 section tires on the Tarmac, thereby gaining back a little smoothness on chipseal. (I like Michelin Pro Optimums a lot.)

I don't expect a big performance difference on flats. I'm hoping to gain a bit in the mountains with a Tarmac, especially on descents.