IMO the air box mod dosen't do anything. The drop in K&N might give you 2-3 hp at most. A AEM Bruteforce intake was added to a titan and then dynoed and it made like 8hp and 13trq to the wheels. It is worth the $ to get an aftermarket intake, again IMO. Best intakes are S&B, AEM, and Injen. Intakes also go nicely with an exhaust and Tuner. I have all 3 and the difference between stock is night and day.
I did the DIY AB mod before buying the S&B. You can tell a very very slight difference in power and noise with just the DIY mod. The S&B is a big difference. Its pretty quiet in normal driving, but when you get on it, it can deafen you (in a good way). HP wise, i dont have any specific numbers, but its def more than the the DIY mod. Well worth the price IMO. they're pretty cheap as well. An as Southside said, all three make a big difference.
S&b has awesome customer service. Had a vibration noise with mine and they are working with me to get it solved with free parts. Highly recommend s&b for the sound and performance. It looks cool also with the clear lid. $208 is what I paid at autoanything.