My Dd was not entered for level 6 SATs but was doing the work but I think it was the first year the school put any children in for it and so only put in 1 child for the maths sat - they were sure would get it. DD was assessed as L6b in maths when she arrived at Yr7 in her GS. So for her it has not made a difference but she did pass the GS test.
DS just managed to fail the GS test but did level 6 reading and maths papers. He passed the maths papers but failed the reading. DS and I had discussed with his teacher that ds was very unlikely to pass the reading test as so few do and he was not mature enough to grasp a lot of what was being asked. His teacher still felt it was worth him trying as he was in the extention group. The fact that he was doing the papers probably helped at his appeal as he is now in a grammar school thanks to the appeal and is doing well. - Apparently doing a maths challenge tomorrow