A stock WRX really shouldn't be racing a C5 Vette. With mods - say a Turboxs stage 2, you could definately give a C5 a good race up to 90mph or so.
A stock WRX if launched properly could hang with a C5 up to 50 mph or so. Thats it. After that the C5 is going to pull away from you like its nothing.
In my Stage 4 WRX, I beat every C5 Vette I ever raced on the street. I even beat one up to 130 mph from a 1st gear roll. Most C5 Vettes run low to mid 13s (on average) at the track.
At our local 1/8 mile, I beat every Z06 Vette I ever raced. I know this is only the 1/8 mile. But being able to beat a Z06 up to 85 mph is worth bragging about. On the street it would have been easier for me. So don't discount someone with a stage 4 saying he's been beating Z06 vettes.