Swaybars increase in stiffness to the 4th power of diameter--There is no reason that I can come up with that suggests that it is good to setup a Focus with a RSB that is over 200% stiffer than an SVT one (21mm). If that is what you need to get your car to 'handle', then perhaps you should really re-evaluate the spring and damper setup being used. A sway bar *should* be a tuning device--like the frosting on the cake...not as the "batter" of the recipe. By their very nature, big sways also reduce the independence on the suspension--how is that good? That goes for any car. It goes doubly for the Focus because of the rear LCA issue...reinforced or not. The amount of leverage generated by the 28mm RSB is well beyond the scope of design for that LCA.
A 25mm is plenty big--and a 28mm is 57% stiffer than that.