Hi,
The R21 or R19 quoted for FG insulated 2X6 walls is figured as though the wall had no studs or other thermal bridges. In reality what you get on a real wall is more like R14. One way to get around this is to use rigid foam board on the outside under the siding -- the foam board has no thermal bridges.
Adding 1 inch rigid foam board takes the R14 up to R18.4.
See the "Whole Wall Heat Loss Calculator" on this page:
Heat loss and energy simulation software
Another good wall is the Mooney wall:
Mooney Wall -- A low cost, high R value wall
The pictures show the Mooney straping applied to a 2X4 stud wall, but you could just as easily use it on a 2X6 stud wall.
I would seriously consider dense packed cellulose in the walls rather than FG. To me, there is no reason to believe that the same kind of convection losses you get in loose fill FG would not occur in bat FG. Its alos takes a whole lot of care to fit FG bats well enough to avoid gaps etc., and most insulation guys are not going to take the time to do it.
Gary