What kind of HP gains - really?

[Mike-in-Orange]

Probably opening the proverbial can of worms here, but I've looked and couldn't find the answers to this, so here goes:

Has anyone done before/after dyno testing on the following to really quantify HP gains:

1. Cold air intake - from an Airaid with the stock intake hose to a BTG with the cone filter and smooth intake tube to a K&N drop in with the brand-x pipe? Anybody actually dyno one or more, and not just quoting the manufacturers claims?
2. Cat/back single or dual exhaust? Someone posted feedback to PT-Tuning that they had installed a cat/back dual and before/after dyno showed a 20HP gain. I find that a bit difficult to believe. What do you guys/girls think/believe/know for sure?

[SteelBlueGT]

Search the topic and U will see there has been a few dyno runs. About 15 hp from either K&N Typhoon or the K&N drop in sans silencers and 3" pipe! But check it out for yourself as there is much more info available!!