Eibach only makes a 27mm for the front for us, which is the same size as stock. Unless they just came out with a new one
Anyway, our cars have a massive amount of understeer (they push in the turns). The reason we upgrade to a larger rear sway bar is to get more oversteer in the car (loosen up the rear end). Upgrading both sway bars would only negate any gains you get out of ugrading just the rear. There are plenty of options for larger rear sway bars.
There is no sense spending 300 bucks for a kit when you can get much better gains out of <150 for one bar and good endlinks.
Sway Bar Info