Given one is older than the other, the WRX has 20,000kms and is running perfect, the liberty has 60,000 and has a full exhaust

The Libery has about 28 hp (21 kW) more horsepower, & weighs only about 100 lbs (45 kg) more. Given that it is slightly older, u can expect a loss of a couple of hp, but having added an exhaust should more than compensate for that. Head to head, the nod goes to the Liberty, assuming ur WRX is completely stock. Without the exhaust, it'd be somewhat of a driver is race with the Liberty still having a slight advantage. Realistically, anything can happen, depending on the drivers. That is what it always comes down to.