The Forester XT has been using the same gears and main shaft as the later WRX. So small tooth size but the wider face to face dimensions on the lower gears.
I don't think the lower final drive gearing really has any effect on the reliability of the individual speed gears themselves. If the 4.44 was used in conjunction with taller gears then it would be a different story. That brings up another good point...a taller gear is going to be less stressed than a shorter gear. Therefore the RA gear will be less stressed in 1st gear than a WRX.
As for final drive ratios from Subaru.... only 3.90, 4.11, and 4.44 have been used in the current Subaru models with 5-speed manual transmission. In the 6-speed only 3.90 and 4.44 have been used. However, the rear differentials are Nissan units so its very likely that alternative rear ratios exist....its just that there probably is not a corresponding front ratio for the gearbox. I do not think, however, that is possible to produce a reliable final drive gear shorter than 4.44 for the transmission.