I went from 172.5 mm cranks to 165 mm cranks, and I vastly prefer my new, shorter cranks. I have long limbs, so with the longer cranks, my thighs were massaging my breakfast when I was in an aero position. Not great.
While I don’t think your reasoning is quite right, your plan is sound: you should opt for slightly easier gear if you want to keep the torque (which determines the force on your joints) approximately the same. When I went from 172.5 mm cranks to 165 mm cranks, I subtracted 2 teeth from my chain ring (I’m rocking a 1x setup on my road bike), and I have been super happy.
But I don’t think you should expect higher speed at the top end: at the end of the day, power is power. Getting shorter cranks doesn’t magically give you more power (unless you were biomechanically limited in some way). Moreover, if you look at the speeds mere mortals reach while pedaling hard, I don’t think that’ll be much faster than 50–55 km/h. And for that, 50:12 or 50:13 is more than enough, assuming you spin at roughly 90–100 rpm. You only need harder gears on the downhill, and even here, the difference isn’t as massive as most people think it is.