In September 2011 (for the first time in living memory
) RGS was actually undersubscribed, so, yes, if you put down the school and achieved the qualifying score you were offered a space. The school did further testing in October 2011 and offered places to another 20-ish boys in January 2012. As far as I am aware this has never happened before.
For September 2012 the usual admission criteria came into play - catchment, distance, brother etc. and not all applicants were offered a place. This link may help http://www.buckscc.gov.uk/bcc/schools/admissions/allocation_profiles.page?