Tough to call. My numerous trips to the drag strip with my 06 WRX has netted the following results:
Not all drivers are created equally. I watched a C6 Z06 run a 13.8 and a 2003 Z06 run a 14.0 at sea level.
Do you have a time the c5 is running? Or a range you'd like to be in? Stock or modded c5?
I can tell you from experience, that a Cobb Stage 2 06 wrx can do a 13.8 with a 1.825 60foot no problem. After a protune, I have no doubt my car as it sits in the parking lot is capable of at least a 13.2, maybe a 12.9999 if the moons align and the car gods are apeased. I also know that a protuned stage 2 STi can run consistent 12.1's-12.2's. (several here do).
For 11.9x or lower, the general formula seems to be 330+ WHP, which is about a (td06h-20g, fp green, sr56, etc.) swap with a protune and supporting mods.
14.xx's-13.xx's = ~ $0-$2000
12.xx's-11.xx's = ~ $2500-$7000
10.xx's-9.xx's = ~ $10,000 +