Can I set a key to increase (or decrease) speed nonlinearly, such as geometric progression (0.25 -> 0.5 -> 1.0)?

[teadrinker2015]

I understand there is a speed list feature, which loop unidirectionally. My proposal is different from the existing one in 2 ways:

• Can be maped with two separate keys to achieve bidirectional action (accelerate & deaccelerate).
  If I want to decrease the speed immediately from a middle position in that "predefined speed list", I have to first increase to a much higher position, which may lead to skipping a large part of the video. The effect is opposite to the purpose.
• Stop at two opposite ends (e.g. 1.0 <-> 4.0), without looping from the start.
  If I want to get to a particular position in that "speed list" as soon as possible, I always have to remember what position I'm currently at, and perform the exact times of key pressing to get there. Exceeding means trying again and wasting a lot of time.