Yeah - It is most likely going to be the separator plate so that's an easy fix. Is this an Auto or MT. If MT, how's the clutch holding up? If it's marginal, suggest changing it out at the same time so you avoid the tear down cost again. Good time to resurface/dress the flywheel surface too since it has to come off to replace the separator plate.
$300 seems a little high for a Subaru shop with qualified techs to R&R the part. Only takes about an hour to get the tranny in an out, two tops and probably another .5 - one hour to repair the separator plate. 3 X $75 hour (usually about $50/$60 an hour for an independent subaru shop) = $225.00. If they do it faster than that, then it's money in their pocket...
Anyway good luck with it and keep us posted.