Firstly, I will say that most children of this age that read/write/do maths etc do those things because they have begun learning them before their peers. Reading at 2 is within the realms of possibility for many children, but most are not exposed to it. These 'accomplishments' are not necessarily indicative of giftedness, and many 'bright' children are capable in this way.
Children that are of above average intelligence (but not gifted) that learn these skills early will have had far more experience than other children in doing them, particularly at key assessment points (reception, y2, Y6 etc). Most other 'above average intelligence' children will have caught up with these early learners by age 11, or 14 or so.
However, that's not to understate the advantage that early reading gives in the classroom in the early years- a child that can already read can assimilate information far quicker that one that is having to use their brain power to read. each. word. in. every. sentence. they. meet.
As children that were extremely able, both DH and I felt let down by the state system- there is no stretch when you are top of the year in every subject, often by 10-15% points. It's highly frustrating, and school is something to be endured until you can get home and learn something!
We put our children in selective independent schools. The benefits for our children are there for us to see every day-to-day they are flying.