Clearly there will be SOME correlation between SATs and school performance and who gets a Grammr place.....in terms of those who achieve BELOW a certain SATS level are unlikely to make it. So I agree that those on the old system achieving 2C at KS1 or 4C at KS2 were unlikely to make it. Likewise, those on L6 at KS2 would have a better chance, but couldn't be guaranteed. Those achieving very poorly and very highly on SATs are also likely to be those lacking or having the vocab and conceptual understanding to not do well or to do well on 11+. However, there will be man you children in the middle - those who were 4A or 5A - and for them, it might be much harder to know if they would or wouldn't get in - some would and some wouldn't from across the range, largely determined by whether they had been prepared or not.
Isn't it daft that in Bucks, the primary and secondary schools, including Grammars and non-Grammars are all part of the state school education system, but the primaries refuse to comment on most likely/suitable school for next stage. Crazy, and contributing to the class gap, where only the middle class kids get prepared. If the primaries did at least some work, those bright working class kids would stand a bit of a chance.
Secondary schools would be slated for not providing advice on unis or careers, but somehow it's okay and encouraged for primaries to pretend grammars don't even exist,in areas where up to a third will go to them. The kids who really lose out from this are the working class bright ones, not the middle classes who will fight for the information, pay for the tutoring or home prep and boost their middling children to the necessary standard, pushing out those who might be cleverer, but not prepared.
So SATs can't tell you for sure, but they can be an indicator of later achievement. I agree that usually it is clear at 5 who are the bright kids and who are less so...and there is some movement across primary, but not as much as you might like. I also agree that early literacy makes a huge difference.......some children are fluent readers at 5, often because their parents have taught them to read and worked hard at it (controversial I know) but once a good reader, even if others do catch up, you never lose your reading ability. I certainly wouldn't be settling for the once a week at school reception reading which often goes on - it's needed every day at home 365 days a year in the early stages. Hmm, sorry,mgone off topic!