So, a panel K&N made no difference, so it wasn't a filter restriction. K&N CAI claims to add 10HP to the 6.0L, which makes 367HP stock, the 4.8L makes 295HP, and your butt dyno isn't picking up 5-10HP, but your ears most certainly pick up the noise. Were there any silencers you could see in the factory intake plumbing? Give it a look over, if there aren't any obvious restrictions; if there aren't any areas that are smaller than the throttle body, then draw your own conclusions
On an otherwise stock engine the potential gains with a CAI just aren't there, particularly not with the tradeoff in poorer filtration. Often times if there are gains to be had it's through the elimination of the silencers that are sometimes fitted to the factory intake plumbing. Removing those can yield the same marginal increase without the loss of factory filter efficiency.
On non-stock engines, it's a completely different discussion but even then, most stock intake setups with the silencers deleted can support a surprising amount of power above stock. Of course this can vary significantly vehicle-to-vehicle and some OEM's aren't generous with airbox and filter size like is the case for most performance cars and trucks, which tend to have large plumbing, large airboxes and large filters.