Ben,
I re-checked my owners manual and it definitely says synthetic oil within the recommended weights (5W-30 or 10W-30) is fine to use in any Impreza engine (2.5i, WRX & STI).
I also heard on WRX forums that synthetic oil is almost 'needed' in the turbo engines if you plan on doing any kind of racing as they run hotter by nature. The oil lines which run around the turbo and are used to cool it, get extremely hot and can cause regular oil to break-down very quickly. However, synthetic oil is much more resistant to these hot temps and will better cool the turbo and lubricate the engine!
-Chike
