I took this three times in the past few days, and didn't notice much difference between the 300 km/h and 430 km/h. I tried to sit at the end of the car so that I could look out the back window, but the view is obstructed. The most interesting part is when you pass the train going in the other direction -- that's when the speed becomes most palpable.

Note that the ride take only a minute or two longer with the slower speed. Maybe they do that to reduce the wear and tear on the system at off-peak times? I am just speculating.