Rich,
Where have you sited the head unit? In the pod on top or in the centre as oem cd unit?
Same place as the OEM was - normal Subaru location. Double DIN fits with no issues.
do the 6.5" and 4" speakers fit in the doors okay?
Yep. I seem to remember I needed a spacer for the fronts, which was a Vauxhall item from memory, but apart from the extra expense (£2.99) it was fine. The standard rears are rubbish, but as virtually nothing goes through them you don't notice much difference when you upgrade.
A decent sub makes the biggest difference to any system - means you don't have to over-drive the fronts to make bass, so can use them as they are supposed to be used.
Have you an OEM sub in the boot sidewall...? You will lose this if you switch from a standard head unit as it has a separate amp channel in it to drive it. You can regain this by adding a small separate amp though obviously. Tis still crap though in comparison to a standard 10 or 12" driven correctly.