Anyone know how to scale rounded rectangles imported from Illustrator?

I took another few minutes this evening to consider the Points Follow Nulls approach to what I think is your desired workflow.

If you have a rectangle path with rounded corners and you use the standard Create Nulls From Paths/Points Follow Nulls script to create nulls for every point, you can add a couple of Slider Controls or a Point Control. to a Null (Control) layer and then add some simple expressions to the null pairs at each corner. The pair of nulls in the top right corner takes the values from the expression controls and adds them to the original values using a simple Value + Effect Control expression.  The top left pair of nulls needs to subtract the X Effect Control value from the original position value and add the Y. The bottom left pair adds the X and subtracts the Y from the Control, and you guessed it, the bottom right subtracts both X and Y from the pair of nulls.

Then if you parent all of the Point nulls to the Control Null (layer-So you can see it in the demo) and lock the Control layers scale to 100% with an expression, you can scale the rectangle with the sliders or point control, and move and rotate it by animating the Control layers position and rotation.

The comp looks something like this:

This should work for any path that has control points. You just have to figure out whether the control handles for the pats are right or left, above or below the center of the shape, and apply the appropriate math to the Expression Control value.

I have uploaded a project file.