Woohoo is spot on. They are not on an even playing field to start with, but no one group (state vs prep) is ahead particularly, rather different strengths and knowledge across the board. It honestly levels by the Christmas holidays as they find their new grooves.
Maths groups are established relatively quickly - after the October half term generally, but these sets are absolutely fluid. My daughter for eg struggles with maths but is not your standard bottom set child. She was told she'd be moving to set 3 (of 6) last year but she has explicitly requested that she stays in the bottom set as she feels more comfortable there, yet now in Y9 she's top sets for all sciences.
GCSEs are nothing like the 11+ race. She's just made her choices, for that is what they are - her choices and not mine, and apart from a discussion with me between two subjects, she's absolutely had the final decision.
Long story short - the 11+ cannot be beaten for the level of angst and stress it causes! I know it sounds so cliched, but your girls/boys will end up at the right school for them and GCSEs will bring nothing near the stress - those on here with older children will (I hope) agree that your 10/11 year old now will be so very different even by the end of Y7, and despite the bad press they sometimes get, most teenagers are bloody lovely and thirsty to learn and do well, so that's what they do.