? what is the outer dia & length of the Impact barrel

Most likely 14mm OD for all calibers. 
Just like the rest of the FX ST barrels in all other guns.
Just replaced the .30 barrel of my Boss walnut with a home-choked 16mm .25LW barrel. Even fitted a 18mm carbon sleeve and it fits nicely. Looks way better to me than the original skinny barrel.
Because the Impact barrel is going through the whole action and barrel supports, it is very likely only the original barrel diameter will fit.