Well, it is a risk either way, but if you want my risk assessment, I think that the highest teams usually will just switch places in their desperate attempts to gain higher rating.If some team gains rating, some other team will lose rating.
Personally if I was 21 of 31 I would just have gambeled that was good enough and sit on my rating, but as everyone else says, only you can decide :)
One way or another, it is an incredible result you have achived with your shamans.