question: is there a point in making the collision radius roughly the size of the sprite? would it slow down the game otherwise or something?
Yes - the AI uses it for certain things, and the first pass of the collision detection algorithm does, too - so that it doesn't have to do detailed, more computationally expensive checks where it's clearly not needed.
You want the collision radius to be the smallest possible, where it still fully contains the collision bounds and the shield. Note that this isn't the same as having it be equal to the shield radius, because the shield center and the rotational center of the ship (which is also the collision bounds center) can be different.
If it's too small, you'll miss some collisions. If it's too big, you'll bog the game down and confuse the AI.
