Yes, spreading out the fire points to a 3 dimensional area instead of a 2 dimensional one should increase hit percentages.
Also, looking at the interior of the X-Wing's S-Foils, you'll see a lot of exposed machinery, especially around the engines, hence the Heat dispersion factor. Remember that in a vacuum, the only method of heat dispersion is via Radiation. Convection and Conduction both need surrounding atmospheres to work, and thus the only way to decrease heat in a spaceship would be via having as much esposed area leading out into space and letting it bleed off (Though, the X-Wing's S-Foils wouldn't do that good of a job of it, the radiators would be better placed on the outside of the wings rather than inside, but hey, I'm defending artistic licence with science, it can't be perfect.)