I just bought a CAADx Ultegra Disc a couple months ago. I absolutely love the bike! I ride 90% on the road the rest on rail trails. Right off the bat I bought a set of Conti Grand Prix 4 Seasons in 28mm size. I mounted them and did some road rides with them. Like them great tires. But.... Last weekend I decided to goto a rail trail. Left my house, road for about 30 minutes on the road, and then hit the rail trail. The stock Sammy Slicks felt so good. And honestly, they roll really well on the pavement. I was probably a little slower with those tires, but if riding solo, they are fine for riding on the road and rail trails. When I ride with others I will throw on the Conti's.
I was thinking about getting a second set of wheels also, but after riding with the sammy's on the road, I don't see a need. I will just switch tires when I want a little faster ride on the road.
You will love your new bike. It's so cool how versatile it is. Perfect geometry for my needs....