I changed my battery on a CS400 out for a Bosch Silver within 2 months of getting the car. Did the same on my last 08STi as well. If you work away for a month or 2 at a time the standard Panasonic won't cut it, even a solar trickle charger from Maplin never helped on either of mine. The Bosch Silver will run and run.
If you get it replaced under warranty, I suspect you'll have the same problem again soon.
I've heard from a dealer that Subaru will agree to change it out once under warranty - but on subsequent failures they'll claim it as a "maintenance issue" rather than a warranty failure.
If I was you - get your dealer to replace under warranty with a Bosch Silver.
I think I used the bigger of the two Bosch S5 following ones bought from Halfords... needs a wee tweak to battery clamp - but my car has sat for over 8 weeks and starts first time every time.
561 400 060 S5 004 (61Ah & 600A)
LxWxH
242x175x175mm
563 400 061 S5 005 (63Ah & 610A)
LxWxH
242x175x190mm
It's a pain in the arse - but a good investment in a hatch to make. The dealer when I traded mine in commented that he noticed I'd upgraded. And when I got the CS400's first service, the guys at the dealer asked if I'd had problems and said it was a good idea to upgrade.