I really think that top speed is way more a sign of the drag of the car than the engine power. Just think that to get only 1 more km/h at >300km/h speeds you need quite a few kw of power. Yet only the opening of DRS or a tow can increase that value way more. Just imagine if they would take out the upper flap of both wings, they'd probably lose lap time but their speed on the straights would greatly improve. Engine power I think is more noticeable by how fast the car accelerates out of the corner. if you look at gps telemetry data comparing different engined cars we can see the best engine reaches peak speed faster but usually the slower one is able to reach almost the same top speed.
Like you can see in this telemetry data after the first corner the blue car seems to accelerate faster in the beginning (even tho he leaves the corner with higher speed I think the distance between blue and red line are getting further apart just after the dotted line) but regardless of that both reach a point that the drag is just so high that they equalize in speed because a difference in 10-20kw brings very little extra speed when you go that fast.