but see, this is an unfair comparison if you're going by used prices or what CAN be done.
You picked the lowest model, cheapest mirage thats 2years older than the oldest WRX. You can pick up a regular impreza brand new around $16k which isn't too far off the mark.
Put your extensively modded mirage against an extensively modded WRX. Professional driver. Now you have your answer.
Autocross isn't that concerned with how much power you're putting down but rather the handling. As soon as the WRX gets swaybars, endlinks, and an anti-lift kit its an entirely new car - through other handling goodies that I'm sure you have in their like drivetrain bushings, springs, struts, strut bars, etc. and the car can compete.
The WRX was made to be a nice daily driver and as a result is too much of a fat pig, just like the Evo is - except mitsu made the Evo less complaint the regular WRX and even the STi so it is more handling driven from the factory.
For the record, a modified widebody STi just took its class and I think an open class in one of the national autocross meets. You don't usually see mirages or other cars doing that.
Consistently the STi is on top of its game, but thats an entirely different animal than my WRX.