Right now the only way to change the rotation speed of a machine based on a redstone signal level are through the use of adjustable chain gearshift but these are only limited to 1 to 2 range.

My suggestion would be the addition of a block that divide the input rotation speed by the level of the redstone power.

For instance, the gearshift receive an input of 255rmp. At 0 redstone it output at 255rmp, at 8 redstone, it output a 128rpm, at 13 it output 34rpm and at 15 it act like a clutch and output 0 rpm.

The formula would be output_speed = input_speed - round((redstone_level / 15) * input_speed )

Of course an inverted version that only transmit rotation when powered is also possible.

Even though it shouldn't break the game balancing, as it can only output rotation speed below the input one, i still thing it would be better if the recipe is relatively late game (Involving brass maybe ?)

Clockwork adds a block called the Resistor that does the sort of what you are looking for. It takes one-fifteenth of the rotational force per strength of redstone.