The tests we have here are different so I can't speak to specific exam performance but in a general sense our DD improved after acceleration. I put it down to a greater interest in what she was learning therefore better retention, increased confidence that went with being happy and engaged, peers working at a higher level encouraging her to strive towards greater expectations.
After acceleration, DD had to move up on national exam competitions and do the ones for her grade not age and I did not expect her to improve as much as she did. Exposure to higher level material at school probably helped but the competition exams are set above grade level so have material they are not necessarily exposed to and she still improved.
It didn't happen immediately, maybe a year after acceleration. If you do see a dip initially, and you may not, don't be concerned. What you really want to be looking for is whether the skip is creating a happy, confident learner. If it is, then the results will follow.
