That's an age old debate....the deeper enrichment, or the faster pacing. We've even had it on here a few times! I personally feel that
both are greatly needed, but if the depth is lacking regardless, the child may as well advance. Hard core numbers are hard to argue.
Developing Math Talent talks well about how to get them and how to use them for decision making.
It's hard to take on that line of thinking though. I was unsuccessful with DD13 back in 6th. I can still remember the teacher telling me that DDthen11 didn't choose to do the extra activities, and therefore didn't need them. DD11 claimed they were just more of the same.
Thankfully the same teacher did realize that DS was "different" when he came along. It found it easier to argue in the extreme. DD13 is and was very talented in math, but in a more "MG/HG" manner with lots of (equally neglected,

) company.
One thing we found once grouping started though is that a group of like minded kids
really helps. DD13 is in great company at the moment. I can only dream though about how things might have gone with grouping from the start,

.