As im currently on my 5th Impreza (currently new shape), I would say with conviction that (in dry conditions) there would be nothing between a 172 & a "standard" WRX........
Now an older Impreza or something "slightly modded", then game over!!!
When I got my current car, I was dissapointed mainly with the delivery of the power which felt like the lack of it. OK now it has been changed beyond recognition with complete decat pipe (all 3 cats out), air filter, 18" wheels, LEDA race suspension. Now its a much better car and is faster than any of my previous Imprezas over a proper road, including the ver V Type R.
Also Impreza engines never loosen up until over 10K miles and that is the absolute truth.
The other problem is that Subaru import them with really restrictive exhausts, so that with only an exhaust change will release the real power of the car,,, but I agree that a standard WRX will struggle against a 172 in the dry.