I believe Meiwa is very nearly as hardy as Marumi, but I am by no means a kumquat expert.
Kumquats can only survive down to zone 8a, and that's only in the South, and they do not do well in the colder part of zone 8a, close to the border of zone 7.
I read of an experiment done close to Atlanta (zone 7b) where someone planted a kumquat, protecting it with a little frame covered by frost cloth, to see if it could survive. The winter killed it.
Is it the hardiest kumquat? Probably. Unless you count Ichangquat (kumquat x Ichang papeda), but that is a pretty hard one to get a hold of, and in addition the skin is said to have some moderate bitterness (although of course nothing like poncirus).
My small Ichangquat seedling appears to have survived outside through a winter in zone 8a, Olympia, WA, although the leaves do not look as good as the Yuzu.
Judging by how it has behaved, I would imagine a kumquat would really struggle up here, but I am not in the South.
There's also Sunquat (kumquat x Satsuma mandarin) but that is not any hardier than Meiwa.