(Just up front: (1) All comparisons made keeping all other factors equal! (2) I know this is a really hard comparison, but there seems to be virtually nothing canonical on here or the rest of google, so I figured it was worth a shot to ask.)
Currently I own a beautiful 80s Gazelle, full chrome, steel frame, on which I occasionally do trips of various distances. I'm struggling to cycle faster than 36 kph avg on it (no aerobars) over say 100k. I have an 8 speed, and I believe my highest gear is 12/54.
Because of this I'm thinking of making an upgrade to a proper aero bike, I think it might make a difference. But how much of an impact can I really expect? I'm looking at the Canyon Aeroad CF SLX 8.0, which I might fit with ENVE 7.8 racing wheels - I'd say that's about as aero as it would get. What kind of (rough) speed improvement can I expect?