It's possible your PV map can be improved, but there's a larger factor at work here: air resistance. The air resistance that works against your bike, or anything else, is much higher for higher speeds. The math equation (simplified to leave out other factors that don't depend on speed) is force = speed x speed (aka f = V2).
For example, the resistance posed by the air at 30 mph = 30 x 30, or 900. At 60 mph, it's 60 x 60 = 3600, or 4 times as much, even though your speed only doubled. The resistance from 60 mph to 80 mph increases from 3600 to 6400--which is why gas mileage drops off so badly at higher speeds.
That's also why it takes huge increases in horsepower to get fairly small increases in top speed, and why streamlining the shape of a vehicle is so important to fuel economy and top speed.