^^^ Actually, there are quite a few guys out there building rally cars that want smaller brakes to use smaller wheels.
I just did all this research.... hours of it.... and it's all correct... although there is some missing info. I'm upgrading to a Rotora BBK in the front, and rear brembos on my 06 WRX.... in under 2 weeks
02-07 WRXs and STI front brakers are interchangable. The brakes all have the exact same mounting holes. Obviously you will need 04STI rotors, or 05-07 STI rotors redrilled to the 5x100 pattern
There are a couple options to deal with the e-brake in the rear.
1) The DBA 2657 rotor will match the rotor hat to your 170mm parking brake. (The STI has a 190mm parking brake)... but it will also maintain the same outer diameter as the STI rotors, so you can mount rear brembos.
2) Godspeed UK sells "extended" parking brake shoes... so you can use stock STI rear rotors (5x100 obviously), but your OEM parking brake will "grab" the inside of the larger rotor hat. I have read about issues with these not working all that well though... so beware.
3) Use stock STI rotors.... and wheel chocks when you park.
Just another little tidbit. The 2002-2005 WRXs have different rear hubs than the 06-07 WRXs. Even if you do buy the DBA rotors, or the Godspeed shoes, you will still need to run an adapter bracket to mount the brembos on the rear.
06-07 WRX and STI rear brakes are interchangable and do not require said bracket. So, even if you decide to go the "cheaper" route and buy the 06-07 WRX red 4/2 pots, you will still need the adapter bracket, although no special rotors will be required, just stock 06-07 WRX rotors.... so you'll save money on the calipers, and the rotors.
You can also run 06-07 red 4/2 pots underneath the stock 16' 02-05 WRX wheels (with a 5mm spacer). That won't happen with Brembos.
Hope this helps.... Good luck.