well, it's an easy choice for 225s. Your rims are 7.0" wide so it should work fine.
Frankly, the stock 215/45/17s are a peculiar choice for Subaru. They use 25" tires on all other Imprezzas including the STi. Your "stock" tire height is really 25" even though the tires that come from the factory are .4" shorter. I think they just tried to keep it different than the STi while still wanting to shift it to 17" rims which lead to them putting on a rather weird tire size choice for 06-07.
It's worth noting that the 08 WRX returned to a 25" tire. It's only those 2 years that got 24.6" stock tires.
Can your rim support 225 wide? Well, it's 7.0" which is 177.8mm which means the tire is 26.5% wider than the rim. BFG I know says keep it 0-20% but most tuners do 25% routinely. Ideally you'd have a 7.5" or 8" rim on there to do 225 wide but if you're going to use a high performance tire with a stiff sidewall, 26.5% should be fine.
225/45/17 on a good high performance stiff tire is what 06-07 guys should go with. You might consider getting new rims though, your stockers are over 23lbs each unlike the other older WRXs (and all STis) so going something like a <18lb 17x7.5" rim would be a hefty 20 lb savings of unsprung weight.
04 STi 16G powered lagless fun
04 WRX: SOLD