These headers will more than make up for any possible loss of torque. I would run a hot dog in the centre of the exhaust to act as a resonator and a glass pack at the rear end of system. 2.25" diameter set up will be near optimal for the 4G54. The catalytic converter should only offer 3-4% restriction on gas flow and have beans all impact on torque. This should be worth about 8-9 HP and some useful extra torque.