EPA (end point adjustment) is used to limit a servo's overall travel. Allowing the servo to attempt to pull more than is necessary can damage the servo. You should set the end-points (which should be on your Acoms 2.4gHz radio) so that the servo moves just enough each way to get full lock.
Expo (Exponential) is used to change the feel of a servo's travel.
Imagine that the green line is where your wheel is. When it moves left, where it crosses one of the curves, is where the servo travels to. Linear means that what you put in is what you get out, whilst the curve (in this case) gives you more travel at first, then less later on for a given movement of the wheel. The Acoms you have doesn't allow for exponential adjustment, so it'll just be the linear option.
I think the problem you just mentioned is that the servo is quite weak, and won't allow for full movement under load. A better servo is something that is often overlooked, but is invaluable as you'll have better, faster control over the car's movement.