Increasing amps can increase speed, because just increasing volts when amps is moderate (20 ish) can mean not enough total watts to reach full potential for that voltage.
Example, 72v 20 amps controller got me 35 mph, while 72v 40 amps could briefly get me to 40 mph. With 9c type 9x7 motor. Doing the math, 1500w from 72v 20 amps simply would not be enough watts to hit 40 mph. It gets harder and harder to get 5 mph more speed the faster you go. 0-25 mph easy, after that, it starts to take a lot of watts.
So its very likely a 60v 40 amps controller will get you more speed than 60v 20 amps.
Conversely, if you hit 30 mph now, a 40 amps 48v controller would not add a lot of speed except on hills or in the wind. 20 amps is enough to go 30 mph, so adding amps won't add much more than 1-2 mph more.
Its just another one of those "it depends on" answers. Adding just amps may not increase top speed much, or any. but having twice the power when you come to a hill, or in the wind, can definitely increase your average speed for an entire ride. So just adding amps can shorten a commute for sure. Faster up hills, faster to get to speed, but much worse range if you ride wide open the whole ride.