Speed is roughly accurate in game, based on the obvious limitations (they can’t model for wind, changeable body position, etc). But versus BestBikeSplit it comes up accurate across different course profiles.
The drafting is far less accurate though (You actually have to work harder to stay in the draft versus real life).
In real life, on the flat, someone doing 3 w/kg at 200w sat behind someone doing 3 w/kg 300 would stay in their draft easily.