At what hp level do the heads become the weakness

I don't think the heads or valvetrain is gonna give out unless your rev the hell out of them! But more like they will limit you as you go up in power. If you got a blower and up the boost the heads are only flow so much before they fall on their face. Alot of people on heer are making 400rwhp+ with stock PI heads. The basics of making power are the faster you can get air in ie blower heads cams, air induction components and the faster you can get it out exhaust longtubes x or h pipe efficiently the faster you will go. So don't worry about the heads, as for goin blown or NA it costs alot more to go faster NA!

as soon as you slap a blower on there, the heads will be limiting power. Port and polished heads along with a blower would net some nice numbers.