i really dont think you need more whp than a viper to beat it. I know most vipers weigh around 3500 pounds, so atleast we have a slight weight advantage. I think its possible for a wrx with around 400 at the wheels to hang or possibly beat a viper.
Gen#3 RT/10 3410 WRX 3100 dif of 310 Lbs
RWHP 450 Stock WHP 400 Stage 4 with a 2.2
or 2.5 conversion
Weight to HP Ratio 7.5 LBS per HP 7.75 LBS per HP
You are riht on the money. The WRX should win that race due to it's gearing and AWD advantage. But a 350 WHP WRX will loose.