I am currently going through the predicted Sats Year 2 - Actual Sats Year 6 at both ends (Year 2 DS, Year 6 DD) and I would say there is actually very little corallation between the two in many cases.
DD was flying high at the end of Year 2 - 3as in everything (and she took the key stage 2 papers not the key stage 1) but she completely messed up her 11 plus and now, in the run up to the Sats, seems to be coming unstitched there as well. Wish it was last year and I could make a trade unionist stand and keep her home next month.
DS is now doing likewise - level 3b already (won't be 7 til the summer) and has been working theoretically 'a year ahead' (it's not a high acheiving school - but lovely pastoral care) since the beginning of Year 1. Fortunately DD's experiences have taught us to be wary. (exDH made the mistake of disparaging the local Comp to our daughter when she was the 'child genius' and we've had to do a lot of back pedalling now she's going there
Basically - kids are unpredictable and just as they grow in fits and starts (and usually just after you've spend £35 on new school schools) they learn and develop the same way. If you want to go down the 11 plus route don't let an average set of Key Stage 1 sats put you off. My DD and I have no regrets and I will probably encourage DS to take the test as well (at the moment he's very keen - he's quite clothes conscious and he likes the GS uniform