As above - is there a kit available for a 55 WRX wagon to convert the rear single piston to the original front 4 pots?

Cheers

 

Out of interest how many pots you got on the front? 4pots~6pots ??

The front brakes take approx 80% of the braking force. To much mussle on the rears and you may well upset the balance of the car. A bias brake control vlv may well be needed to calm things down a bit.
Personally i'd consider upgrading the rear pads, change all hoses for braids if you haven't done so all ready and see how that goes. Still not happy then perhaps a grooved or grooved and spotted disc.

On my classic i've changed the std 2 pot frts for 6pots and grooved disc, Ferodo pads. The rears i've replaced the om pads for an upgrade and left the disc alone. Braids alround and decent fluid. Stops on a six pence