Yes, drivetrain acceleration is a function of current velocity. If you're familiar enough with calculus, you'd recognize this as a differential equation.
Here's a Maple sheet calculating the equations of motion for a typical FRC drivetrain and then optimizing a gear ratio for a specific sprint speed. Our team has developed something similar with SageMath that we will likely be using for ratio selection this year. You're particularly asking about just the beginning section of this but I went in further.
https://drive.google.com/file/d/0B_l...ew?usp=sharing
The equations are very complicated, but check out the bottom graphs. They demonstrate velocity and position as a function of time. The basic is this: higher gear ratios allow for faster acceleration, but lower max speed, and vice versa. The optimal gear ratio for a sprint is somewhere in the middle.
This is using some intro-college level mathematics, and software to solve the equations (these are not trivial to do by hand; in fact, they cannot be solved symbolically, only numerically), so you could see pretty quickly why this isn't done very oftenly.
Oh, and as stated, this doesn't even factor in brownouts.
