This is still visually contradicted, GROSSLY rather than subtly, by the fact that Spock keeps the butt of his ship pointed at Kirok's planet throughout the journey. The visuals really do their damnedest to establish that Spock isn't lifting a finger to get to the planet early. Which is exactly why McCoy is one-and-a-half steps away from strangling him.
Okay, that's if you take the fx literally. But as you know, the "ship traveling backwards" shot was really just the asteroid rendezvous shot being played in reverse, due to budget and time limitations.
But I can still pull this chestnut out of the fire. Let's consider that the Enterprise was seriously whipped after it's battle with the asteroid. So they used an impulse engine burn to get on course for the planet, barely ahead of the asteroid.
When they finally arrive at the planet, they're going to need another impulse burn to slow down and settle into orbit. If the impulse engines are nearing their last gasp, Spock has to save half of their fuel for slowing down, which means he is not at liberty to just keep accelerating at will. He can get up to a certain speed, whatever half the fuel will give him, and that's it.
Therefore, in the backward footage, the Enterprise is coasting toward the planet at all available speed, during which time the bow can point in any direction they like. They could even do an Apollo-to-the-moon style barbeque roll to distribute the sun's rays evenly on the hull. [I say they would never waste precious maneuvering thruster fuel trying to eek out a few seconds of advantage. The cost-reward analysis is terrible.]