Rosa, I would say that the Level 6 thing depends on what the Level 6 is in, because of the fact that the test for Pates is verbal reasoning only.
Therefore children who are level 6 for maths might not get in, whereas those who are that level in English might be more likely to, IYSWIM. Certainly lots of children who gain level 6 fail to get in.
From observation, there seem to be 3 groups of children who get in
a) Those who attend one of the 'specialist' private primaries in the area, who offer a very ... thorough .. preparation for the test, not only in terms of actual VR but also through general exam prep etc. Many of these are also tutored.
b) Oustandingly bright children, from a very wide area, who have done a reasonable amount of preparation at home or at a tutor.
c) Other children who are very heavily tutored.
I believe it to be a fabulous school for those children who get in 'relatively easily'. I would have more doubts for a 'borderline' child.
Remember that you lose nothing by putting Pates on your form as a school to share 11+ results with - if you gain a qualifying mark, that's great and you can put it on your main application form. If you don't, it doesn't get in the way of getting a place in another grammar or in a comprehensive. DS's application had a comprehensive first, with a grammar as an insurance place as we live on the edge of a catchment. He's started at the comprehensive and is having a ball.
[Bytheway we must be fairly near neighbours]