This is a really tough question to answer.
There is no limit to improvement.
A lot of points can be gained by learning concepts in maths and SC that you just didn't know.
Moreover how you handle the pacing on the test also matters. You may know everything, but if you don't manage time on the test properly you may not score well. For example, when I gave my first test, I spent like 7-8 minutes on one single question, and so I obviously lost a lot of points on the later questions.
I think by working hard, going through the relevant books, being regular in your studies, using this forum properly and being smart on your test pacing, you can do wonders to your score.
The only limits are those that we put on ourselves!!