Grade skip increases competitiveness because kids need to "prove" themselves in a more competitive environment - especially so for a gifted child who has been coasting all his life because everything came easily to him in academics. And a side effect is the high level of self confidence the success in a more difficult environment instills because the 6 year old has proved to himself that he is as good as an 8 year old.
If a child is not going to be bored out of his wits all day long and will be challenged at school, then I would go for that opportunity. I would be excited and happy for my child and not worry about what the future brings.
One word of caution though - when a gifted kid is grade skipped they are bound to face challenges because they are not used to doing difficult things. More so if they are perfectionists who give up at the first signs of failure. A parent's job is to keep a close eye on the child at those times and gently prod them in the right direction.
