Real differences between racing and endurance bikes if geometry is similar
I've been getting serious about riding this year, and plan to train over the winter with an indoor trainer and I'm working to drop some lbs. I can hang with 18mph group rides and have done 2 centuries this year, but want to join the fast 21-22 mph group rides and do at least 4 centuries next year.
I'm looking to buy a dedicated road bike, preferably carbon that can run true 32mm tires. I already have a Topstone that's great, but I'd like to keep that set up with big tires and a rear rack.
My head is spinning with all the choices out there and I'm 6'5" so no local shops have anything in my size in stock. My Topstone is an XL, with 40.2 cm reach, 120 mm stem, and 64 cm stack. The Supersix is supposed to be a fast, race type road bike and the 62 cm has a 40.6 reach and 63.4 cm stack. That seems pretty close to my Topstone which is supposed to be a more relaxed, endurance geometry.
What would make the Supersix faster? More aero tube shapes and less weight? How much faster would this be vs an endurance bike at the same weight like a Synapse? It seems to me based on the measurements that you could get a similar fit on either, or am I missing something?