I've read before of people inserting things into lines to increase resistance. Never weed wacker cable though. Sometimes the simplest idea is the best idea!
I just go with the ol' rule of thumb...
One foot of 3/16" tubing for every pound of co2. Trim back until perfect. Works every time! Even for a beer gun.
Crazy isn't it? The only real advantage is, I think, a shorter line. I've seen people recommend line lengths upwards of 8m. Given how cheap tubing is, that may well be the way to go.