This policy brief (pdf) from Chloe Gossage has a misleading title–"Charter Schools : Why More Study is Needed". Chloe is not saying we should keep the cap, I wouldn’t think. Rather, I take her to be saying: if you’re going to study it some more, here’s how to do it. It’s a very delicate way of critiquing the NCCPPR study.
(Read some of my thoughts on charter schools. Although I’m unabashedly for vouchers — even full privatization.)