I haven't heard that SG is not going to a grammar.... I will ask around...it is St. Bernards that is possibly merging with ST Josephs and so giving up its 'Grammar' status.
The 11+ exam is set by NFER.... exactly the same as the Consortium..... but consists of a Maths section (as per the consortium up to this year).
What stats are you referring to. SG's GCSE' stats have gone up over the last 3 years and I believe was around 96-97% A* - C for this year. In fact all of one Maths class (33) achieved A*'s.
I know SG's A level stats are not as good as Langley or Herschel but since many of SG's top students opt for the IB it does bring the A' level stats down as IB is not included but the total number of pupils are. (There is also the fact that SG has an large intake at sixth form from other schools in the area.... so their A'level stats are not just from their own 'grammar' students)
As I have said before..... Langley has always been harder to get in to... Herschel (I believe) is the school that has shown the most progress in recent years......
Slough G has an 'Old school' head who certainly doesn't seem to want to 'market' the school..... but having had an 'Outstanding' OFSTED this year I don't think that you could say it is on a par with 'local' schools. OFSTED would not give an 'outstanding' grade without the pupils having made excellent progress from their starting point in Y7.
I would be interested in knowing what stats you are referring..... as I am always looking to update my knowledge on the grammar schools in Slough.