I have been pondering this a lot lately.
I have 2 dcs. One (dd - 7) is (according to her teachers) bang on average and struggles with abstract concepts and sometimes has no idea what the lesson was about but is quiet and well behaved. The other (ds - 8) is (according to an ed psych) highly gifted and functions multiple years ahead of his peers and is loud and confident.
Ds gets extension work in a group with just him and one child from the year above. Whilst good, I am not actually sure what this achieves - as with many gifted children he is self-motivating to learn and discover things and would probably get as much out of being given a text book to sit and read and work through or better still, access to a computer. The extra help will not (probably) help him in the long term be a happier person or get a better job.
As far as I am concerned, the resources which seem to be available to ds would be better deployed in helping the average children (like dd) who seem to get nothing extra and often struggle on unobserved. The help might well make a difference to whether dd remains engaged with maths or not and therefore have a life changing impact for her.
In general, I don't think we focus enough on the "average child".