The 2011 nav system was 7" for the WRX and STI.
The 2012 Legacy nav system is 8".
Maybe it's a smaller version of the Lagacy's setup, maybe it's an updated version of the Kenwood 7", maybe it's all new.
The mounting from year to year should be the same ('08-'12).
You'll have to ask your dealer if they are able to install the newer nav in the older car. Other than cost I don't see any reason why they wouldn't.
Quality aftermarket units can be had for less than the $1000 price of the stock unit.
I'm sure I'm not the only one but I have a thing with my car being completely stock until its warranty is up.
In any case, I'm much interested in the OP's original question about the nav. Is it good essentially :O ?