Actually there is still a warranty. Powertrain is 5 year/60k miles. Doesn't hurt to try no matter your mods. Mastro is said to be mod friendly.

Straight off Subaru.com
(SUBARU.COM : TOOLS : WARRANTIES FOR 2005)

Powertrain Limited Warranty
Powertrain coverage for all models is five years or 60,000 miles, whichever comes first. It covers the major powertrain components listed below:*

Engine block and all internal parts
Cylinder heads and valve train
Oil pump, oil pan
Timing belts or gears and covers
Water pump
Flywheel
Intake manifold
Oil seals and gaskets
Turbocharger Assembly
Transaxle case and internal parts
Transaxle seals and gaskets
Axle shafts and constant velocity joints (except boots)**
Torque converter
Electronic transmission control unit
Transaxle seals and gaskets
Propeller shaft
Wheel bearings