taking a wild shot here, let me know if i am barking up the wrong tree; here goes.
if the stallspeed formula = sqrt ((2 x W) / (S x rho x Clmax))
there is no Cd in there, W doesn't change nor do rho, and S. The only question is does Clmax change?
Assuming it doesn't, this means that there is no change of the stallspeed.
If it would have a negative influence on the Clmax, the stallspeed would increase.