I don't think it's worth it, to be honest. I mean, it's pretty unlikely you'll end up going up a grade in Biology and Physics and although it's more likely with chemistry, it's still a stretch considering how specific the mark scheme is. As previously mentioned, it's not like English etc. where the marks are a lot more subjective. Good luck though, if you do go for it and congratulations on your results!
Just focus on getting your A's and A*s for you A Levels, that's more important anyway.