You shouldn't pick a tire size that's more than 25% wider than your rim. Not sure if your rims are 6.5" (05 WRX) or 7.0" though, they could be different. Here's a chart I made for tire widths based on rim width:
6.5" width rim: 185 to 215 tire width
7.0" width rim: 195 to 225 tire width
7.5" width rim: 205 to 235 tire width
8.0" width rim: 225 to 255 tire width
8.5" width rim: 235 to 265 tire width
You need new tires when the tread depth starts to get low. There is no set number, that matters greatly on the type of tire it is. A UHP summer-only tire can run with much less tread than, say, a winter tire. 4/32 is the standard minimum for inspections and it represents a good number for all season tires but really if you want good winter handling, you aught to have more and similarly if you are in the heat of the summer, you can get away with a 32nd or two more wear before getting rid of them.
A tire would be "dead" approximately (mostly pulled out of my ass)
Summer Only: 2/32
All Season: 4/32
A note though that wet handling on some tires is not a problem at low tread depths and other it's a huge deal. Hydroplaning resistance is especially critical as you increase tire width and it is directly linked to tread depth HOWEVER! The layout of the tread matters a great deal. If you take a winter tire and run it down to 2/32, it's not going to push water to the sides very well but something like a KDW2's tread would probably still work decently.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD