The struggle with my guy is that he has a number of emotional things going on - perfectionism (had a total shutdown in art class because he is "terrible - no horrible Mommy!" at art, anxiety (nailbiting, incessant hair-twirling), occasional outbursts ("You are telling me this is going to be required by TWELVE YEARS!"), etc. I worry that exposing him to that higher peer group will hurt, not help.
My dd12 had significant perfectionism going prior to her grade skip (she skipped 5th). She took the EXPLORE that year (4th) and came out crying. She also did worst on her best subject (reading) b/c she was so blown away that she didn't know everything immediately as she usually did in reading that she left the test three times to go cry in the bathroom and didn't finish the reading piece by a long shot. She was still a bit above average for an 8th grader on reading, though, if I recall correctly. Overall, I'd say that being exposed to material that isn't all a cake walk for her and having friends who are better than her in some areas has significantly decreased her expectation of herself that she should be the best at everything with no work.