As far as I am aware, grammar schools do not get more funding than comprehensives or high schools. My understanding is that the advantage to the children is being taught st a depth and pace that suits them which wouldn't be possible in a mixed ability situation.
This doesn't sound like unfair advantage, but rather teaching appropriately to children's capability. The only injustice could be not offering all children capable and keen a place.
Unless "cutting off the heads of the too tall flowers" in the name of equal outcomes is the goal. In which case those with the cash flow for private school will be the only ones to escape while middle income and down families will be stymied.
I accept that special needs students can need special provision and sometimes even more funding and it is right for our taxes to support this, even if a child has profound needs that mean they will never be economic comtributors. Why shouldn't suitable provision be provided for the other end too?
Finally, it's my experience locally that more than just the top 2% are being left to coast. I'd say that the top 10-15% are pretty bored.