I knew an Etonian for a brief moment in time. He was relatively 'normal' in that they 'only' had one live in servant but he was studying alongside kids from blue blood families, both foreign and domestic. I don't doubt that the networks he developed will open/have opened doors for him socially and professionally.
However, if you look at the various league tables, you will see that academically Eton and Harrow aren't among the best of the best. But then, looking at it another way, if you went to school with the son of BP's CEO or the next Earl of Whatever then I don't think that less-than-stellar A level grades is going to hold your career back