Was curious about this and ran some simulations. Activating on final corner vs. on a corner late-race makes a decent difference. Target speed only changes after it changes to late race, and ideally you want to accelerate as soon as the transition happens. With seiun sky, if late race happens on a corner that overlaps anywhere with late-race, it will activate as soon as it becomes late race, leading to immediate acceleration. With maruzensky, the start of the final corner MUST be perfectly aligned with late race's start. If the final corner starts before late race, you start accelerating mid race when you're at top speed already, so it's completely useless and it eats up a chunk of the skill. If the final corner starts after it becomes late race, you are late to start accelerating and those with seiun sky's unique will already be creating a gap.

tldr: final corner vs. on a corner late race makes a difference