you could go much faster on the road if you hadn’t set the bar and stem up in the least aerodynamic position possible
Aye, but you can’t buy a different shaped body. Speaking as a man who is apparently (according to his physio) quite flexible for a 39 year old male cyclist, I still have my bars reasonably high on my road bike, because that’s what shape I am, longer legs, shorter upper body.
So assuming the position of the bars is ‘fixed’ then you might as well focus on gains elsewhere, aero wheels, whatever.
edit:
I think disc brakes have just added a bit of weight to all road bikes.
Yep, 9-10kg seems par for the course now for bikes up to and including £2.5k it seems. I think Trek were pushing one of their Emondas (the endurance one) at over 10kg for £2200 (this was a year or two ago right enough, I just randomly remember that example).