apexi, You guessed right. have 215/50/17s on the new wheels. The OEM were 205/60/16s !!!. I have not been able to check my OEM donut spare yet. (just had knee surgery). But judging from the chart you posted, it seems the overall circumference is about .2 inches LESS with the OEM wheels andd tires vs. the new 215/50/17s on the upgraded Subaru 17" wheels. That should be alright for using the Donut spare for emergencies.
To ssbtech: I don't think that Subaru would put a Donut Spare for use in a flat tire emergency (granted for limited use and limited speeds 50mph MAX), where it would damage the AWD drive train in any way. Donuts Spares are for very limited usage on all cars. They were created for weight reduction, more trunk space available, and for emergency usage only until you can get to a real tire change and repair or buy a new tire if that's required. I wouldn't drive for any great distances.
From apexi's chart it appears that the slight variance won't matter for emergency usage and doesn't require upgrading to different Donut Spare. for those of you have 17" wheels on 2010-2012 Subaru Legacy (the 2013 Legacy premiums have upped the standard wheel size to 17"), I'm curious if the Donut Spare is the same one used for 16" OEM wheels...