I think duane v hit the nail on the head. The 4-cam cars will respond even better than a 5.0 to a cam change, as long as the rest of the engine is set up for it.
Take for example a stock 5.0 pushrod motor and throw a cam in it... you won't get much unless you optimise the rest of the system with heads and an intake. Same goes for a stock 4.6.
Now look in my signature to see what happens when you make a 4.6 into a 5.0 liter displacement and port the stock heads, re-grind the stock cams, and re-work the stock intake manifold. Not to many 5.0 pushrod motors making that power with all the available aftermarket, and non even close to that power by re-working the stock stuff.