I would argue that money spent on a cylinder head should be the priority of everybody seeking big power.
Sure you can make some very respectable numbers on standard rb heads by virtue of a pretty good design to begin with but real power gains come from correctly modified heads. Getting the engines breathing characteristics to suit the other mods is the key to unleashing the true potential of your build.
Every performance category be it, drags, marine, Tractor pulls, circuit,N/A Forced induction or whatever has had countless hours of time and money poured in to developing cylinder heads simply because that is where Real horsepower benefits and torque characteristics to suit the application can be found.