A small motor of say 60 watts has a huge impact but mainly on steep climbs when climbing dominates the equation and not aerodynamics (speed). This is because the power needed is proportional to the vertical climb speed while being the third power of the horizontal speed. In practice this means that a rider who does a 400 watt effort will have a 4.5% horizontal speed increase and a 15% vertical speed increase vs an other rider doing 400 watt. As aero is more complicated (the other might have a lower Cx, drafting, head winds etc) it is much less effective to get a boost on the flat. In an acceleration of 1200 watt the effectiveness even decreases to a 1.5% speed differential.
So basically adding 60 watts during a steep climb, or about 1 watt/kg, is the reliable method to create a huge gap in a short time. A rider who has that 'skill' would try to gap the others on steep (and preferably long sections). For such a rider, considering he is already one of the best, it would be easier to win RVV (steep) than MSR (retavily long but not steep). LBL and Lombardy would suit him best and are basically a no brainer as he will win without crashing or technical issues. The effort of winning the Fleche would feel like driving to the bakery. In races such as the TdF he would gain minutes on the climbs. PR would be a lottery as he cannot use his motor as effectively. Mountain TTs would be reliable wins while flat TTs depend on aerodynamics and would be more difficult to win. It's also an other bike. The risk-benefit ratio may not be good enough to use a motor in those conditions.