i read in 1 of the mustang mags where they did a test on a 4.6 car and gained only like 1 or 2 hp. hardly woth the effort. they listed the dyno results so it's kinda hard to argue with them.
also did you see that story on the company that is fixing the problem with the old style 4.6 heads? once done they out flowed the newer heads.i think they wanted under a thousand plus your old heads.