YEp you're right, that's in the charge air circuit, post turbo and post intercooler and right before intake manifold. It makes perfect sense that you got a gain in power after patching that up, because you were loosing boost pressure. I don't know about sourcing, but I think this hose is pretty common, so NAPA should be a good source if you get a guy who knows half a thing or two. Take him out to see the truck, have him measure diameter etc.
So-- is the truck auto or manual tranny? From what I've read, the auto trannies are limited in their mpgs. All trucks though benefit from cold air and free flowing intake. There is a cheap DIY mode with a NAPA 6637 filter (you can do a search) that will do the trick for about $60 total (depending on a few things...).
Next is to get the exhaust free-flowing by deleting the Exhaust Back Pressure Valve (ITP Diesel, LLC
has a nice delete part/'kit')-- but consider that this takes away a factory system to help the truck warm up faster. Once you do that, an aftermarket exhaust, including maybe a cat delete, will help the 7.3 breath on the other end better.
Next, or at the same time, is to change the dead-head design of both the high oil pressure and the fuel circuit in the heads by installing kits that cross over the pressures from one head to another, and that give you a regulated fuel return to the tank. ITPDiesel has a nice looking oil pressure crossover pipe, and Dino Fuel Alternatives has a cheap regulated return with a fuel pressure gauge tap ready to go. There's a better regulated return out there, can't remember the vendor, but it's way more expensive than DFA.
Finally, once you have gauges for Boost, EGTs, and for me Fuel Pressure (if auto tranny, then probably auto tranny fluid temp), you can get a 6 position chip and put an economy tune on 1 position for most of your driving to save fuel.
Since I have a manual tranny, based on what I've read here, I should be able to push my truck over 20 mpg highway with this strategy. Auto trannies as somebody said are looking at more like 18-20 mpg. All of this falls off the table when loaded/towing/in town/climbing hills/off road.
Again, I'm a noob and this is just based on what I've gleaned from reading the ORG for a couple months. So others with more knowledge and experience will probably correct me.
I bet you got a great deal on your truck and you will be able to make it yours. Sounds like the PO was good with maintenance, but there's nothing better than starting fresh with new fluids and filters. In particular, unless you have a receipt or something for the fuel filter being changed within the last 15K miles, change it out and see if it helps the mpgs.