I was checking these out the other day but man they are $$$$$$$
Diesel Performance
Get the injectors directly from a rebuilder, that site is selling someone else's injectors, you'll get better customer service (quicker too) from a builder not to mention better prices. They also do not mention what the core charge is that is included with the almost 4k price tag and do not tell you what it is at least as far as I went in the checkout process. Also since you are buying from a middle man I would expect a bit of a markup.
what i do not understand though, is how they can claim 100s+ more horsepower AND better mileage. As far as I know there is no way to get more horsepower without burning more fuel, unless you magically remove all power lost to heat and friction, which I kind of doubt injectors could accomplish
Better mileage comes from better programming that you're going to get with the injectors, provided you drive with the same foot you do now with better programming (injectors or not) you will get better mileage (presuming your foot is not on the floor board all the time), how much depends on driving style. The injectors may increase mileage if yours are worn already but that increase would be negligible unless yours are extremely worn.
Here's what I'm running in mine now from Bean's or buy new ones for around $1900. The core charge deal works out well in your favor, out of the 16 injectors I sent to bean I had less than $100 in repairs that were needed to bring them up to par for exchange. this all depends on how well the preventative maint has been kept up on the truck especially the fuel filter.