The question we need an answer to is: at a given speed, what percentage of energy used is moving it forward? It won't be a perfect bell curve.
No. But it will resemble one. Aircraft produce lift through forward movement. However, MPs only produce lift through their fan blades turning. As you note, at a hover, no forward momentum is being generated, so 100% of the energy is being used to keep the craft aloft. However, due to the need for the Mavic to self-correct and maintain position, this isn't terribly efficient, as the motors must speed up/slow down to compensate for positional changes. This makes hovering pretty inefficient and likely why battery use during a hover appears anecdotally to use up more battery than does directional flying. These correctional changes don't really happen much when the Mavic is moving forward, as there's no need to maintain a precise position and therefore motors can spin at constant RPMs (efficient).
However, for the MP to move forward (or any quadcopter), it must tilt its body to gain an angle of attack difference and lower the front motors while raising the back motors. Faster forward speeds require a higher angle of attack. As the angle increases, the speed can increase (since more of the motor speed can be used to move the aircraft forward) but this also increases the drag placed on the craft, since the forward face of the craft displays more surface area when tilted. (There's also a number of issues with airflow and motors but that's not critical for our discussion). It also requires more fan blade/motor speed, because unlike airplanes, none of the forward motion can really be turned into lift (in fact, only drag). At higher motor speeds, the motors lose efficiency (energy consumption-wise) and drag is also increasing. At that point, the energy efficiency goes down and the aircraft travels less distance.
Where this precise point is, only DJI engineers know for sure, as the rest is only conjecture and anecdotal experience from users. However, since DJI has the motivation to maximize both speed and "time aloft", I'm making an assumption that they've figured it out and when they market their "27-30 minutes of flight time", they're talking about P mode, not Sports mode. There are likely 2 reasons behind having separate P modes and S modes for the Mavic. One is related to sensor functions (too fast and you can't stop in time for object avoidance, for example) and the second is likely to maximize flight times while allowing those wishing to speed along, get their thrills too by switching to Sports mode. Otherwise, why have separate modes?
DJI engineers probably figure that most people flying the craft in either mode likely peg the thrust (right stick) to maximum when moving forward over distance and therefore cap the top speed for P mode at this maximum efficiency. Anyone switching to Sports mode can go faster but with the understanding that they're sacrificing time aloft for the higher speeds. I'm assuming S mode is limited too, based on the need to keep motor RPMs at a manageable and non-destructive rate. Again, this is all only a guess but from an engineering perspective, it's an educated guess.
Feel free to fly however you want. I'm just presenting my opinion to answer a question that the OP had about the most efficient way to fly.
