Scotty does have a valid point. Aside from the need for maintenance which many owners may not be aware of, the batteries in the MT WRX have marginal cranking capacity when new and certainly don't get better with age. 350CCA doesn't cut it when the temps dip into the single numbers. Factor in low fluid level plus some normal aging of the battery and you have slow cranking and no-starts.
Almost any aftermarket battery down to the cheapest one you can get at Wal-Mart will probably have more capacity than the OEM battery. It's possible that the newer models have better batteries (I've seen new STi's and the battery was a maintenance free unit with 500CCA IIRC), but the 02's had the crappy ones.
I replaced mine after less than a year with an Optima because I wasn't happy with the winter cranking performance and didn't feel like waiting for it to crap out totally.