I'm pretty sure my battery is the original fitment from when new and it's going on 5 years old and takes about 3secs to crank over the engine after parking overnight. I'm going to uprate it anyway as I'm moving the stereo from my old car into it.
The 2.5i has a pathetic sized batteru so I am going to fit a battery tray from the 3.0RB which allows much bigger batteries, and aftermarket tiedown rods + holder $10.00 from Supercheap. Will also upgrade the stock battery>earth cables to 2G and it should be fine for what I need.
Like Smythie said, the original spec does its job and is a good compromise between weight/dimensions/performance but when when you start adding load to the system it won't cope for long.