you can use sti cams in wrx heads
that's what i did for a while. same boat. 257 block, rex heads, and i used usdm sti cams in the rex heads... just seal off the ends of the cams to block the avcs stuff since it won't be there anyway. if you don't oil will just leak out and all over your timing belt. they are a perfect fit; no modification to the head itself is necessary, and they do run a slightly more aggressive profile.
i also like the looks of the new helix cams.
with that setup and a nice big snail you can make over 300whp. the guy that owned that motor before i did made 330whp 350tq on a dynojet using a pe1820 clone.
either way, you're going to like the setup. it's nice and has torque for days.