The USDM WRX drivetrain indeed has different ratios for the two diffs. As long as you get both the front and rear diff from a USDM WRX, you won't have to worry.
If you want to go nutty with crazy short gearing, the 4.44 ratios can be found in a bunch of the USDM automatic equipped Subarus. Since all of the R160 diffs can be fitted with these ring & pinion sets, you just have to find the parts and have a qualified mechanic install them. And while the diff is out, grab a Quaffe, KAZZ, STi, or USDM LSD.
If you have the choice of which tranny to buy, consider these points. The push style clutch trannys found in the N/A cars are cheaper than the pull sytle clutch trannys found in the turbo cars. However, once you upgrade the clutch, pull style trannys will require less clutch pedal effort because of their design. Ever drive in major city rush hour traffic with a big nasty clutch? It gets old after a while.
Assuming your Impreza L is already a 5spd, make sure the WRX tranny you are purchasing comes with the hydraulic clutch actuator (the plunger which pushes on the clutch fork). It's not an expensive piece, but it'll save you a trip to the junk yard if it's already there. If you're just upgrading the drivetrain, make sure to get the flywheel that came with the WRX tranny because the pull style clutches use a different bolt pattern on the flywheel for the pressure plate.