I was talking to a Mitsu motorhead at the Subie Show ... and he uses a small ride-able lawn mower battery in his MR2 personal car ... gets a new one every year.

Would this work for a WRX ... in warm southern CA ... in a wgn without lots of heavy duty ICE ...

Unless you use your car for serious competition and the weight saving would be that important I don't recommend using a lightweight battery. Cranking and reserve capacity will be even worse than the already crappy stock battery. Even without high-drawing electronics in the car the alternator has trouble keeping up with high accessory loads at idle so you need a good battery to take up the load during extended idling periods such as in heavy traffic.