The benefit provided by the output of the motor won't be linear, at least not for road races (cyclocross I'll grant is an entirely different beast).
The ratio of power output to speed follows a square law, so if you're going 20 mph and flip on a motor that doubles your power output, you won't even hit 30 mph, let alone 40mph (4x power = 2x speed). But this is an endurance sport and a huge part of your strategy involves dealing with wind resistance (by drafting) so even a little boost means that you might be able to pull off or prevent a breakaway at a critical moment of a race. For much of the race your goal isn't to go faster than the other guys, but merely to match speed without wearing yourself out.
The ratio of power output to speed follows a square law, so if you're going 20 mph and flip on a motor that doubles your power output, you won't even hit 30 mph, let alone 40mph (4x power = 2x speed). But this is an endurance sport and a huge part of your strategy involves dealing with wind resistance (by drafting) so even a little boost means that you might be able to pull off or prevent a breakaway at a critical moment of a race. For much of the race your goal isn't to go faster than the other guys, but merely to match speed without wearing yourself out.