The games run at exactly the same speed... regardless of the amstrad Hz
If games set the crtc registers they will not work with 60Hz, because they will reset the screen refresh back to 50Hz.
Essentially yes the game code takes the same time *but* if the game is syncing with the vsync then there could be less wasted time.
potentially games could run smoother or slightly faster at 60Hz, but it depends.
It depends on if there is wasted frame time.
e.g.
a game which runs at 25Hz (because it is slower than 50Hz, but has some wasted time until the next vsync and waits for a vsync for displaying graphics).
Could run at 30Hz (if the time the game takes is <2 frames at 60hz).
So effectively the game is running faster and smoother.
Really depends on what time the game is using.