Rode a 8r last year. I'm on 10r this year. Oh my god. Huge upgrade! The Expert this year is better than last years Expert so the jump should be more noticeable from 6r to 8r. Besides that, it's also going from 105 to Ultegra.

6r has a monostay, Expert doesn't. Is that noticeable? Probably not except a little weight. If you test ride them, don't just putz around a parking lot. CRANK on pedals as hard as you can to feel the difference.

Also, the Tarmac isn't a super smooth ride anyways. That's not its intention. Want to smoothen out the ride get a Roubaix. It can be rough going on the SL... put new wheels on it just to soften the road a little.