JA, that could be a problem. We motivated it to the examiners twice that we were flying in from Knysna (we wrote in Pta) and if they could please accommodate us on the day and both times they did so on the Saturday.
Instead of paying for an extra test, why don't you try and book the online IELTS test in Joburg and spend the extra money on a plain ticket? Apparently there are more dates and results are out quicker. Also, immigration doesn't happen overnight so in my opinion don't rush anything, getting your final PR documents will take months anyway.