Righto, I had the same problem with my MY00 WRX when running 98 fuel with the factory ECU. It would take a few extra cranks to fire up, and then fart on 2 cylinders, and then fire on all 4, and then it would be fine. I switched back to premium unleaded (95 RON) and it would fire up just fine! Reality was, there was no power difference in running 95 or 98, it was all in my head, but starting the engine was embarassing if someone was in ear shot because it would fluff about like it was worn and old, and I put it down to 98 fuel.

Try the battery, but also try 95 RON premium and let me know how you go? It made mine start a whole lot better!