well it depends on the rest of the engine combo ofcourse but on something with a stock cam i think ported ( a good port job ) e7's are supposed to be good for 20 or 25 hp over stock. that doesnt mean if some one puts bigger valves in them and ports the hell outa them they wont make more thats just an average from a " decent" port job. gt 40 heads and heads in that class are probably good for in the 30;s hp wise, and the after market aluminum stuff probably 40 and up, which is where the rest of the engine really comes into play.
i have a 65 stang with ported iron heads, 194 160 valves, big cam, and it runs pretty dang good for iron heads.........it's really a matter of finding a good deal on them. hard to spend alot ( more than 5 or 600 ) on stockers being done up when the real good heads are a few hundred more ( unless their afr's ofcourse )..........