These injectors are built to stock specifications, and are the same specifications as the injectors that were originally in the truck. A decrease in power at high end should not be an issue with an injector. A lack in power due to an injector would be felt throughout the power band, and not just present at the top end. The most prevalent injector issues in a common rail system are a no start condition and an engine miss.
Generally with a Duramax when you see a lack of power at high end as you are describing, it is due to lack of fuel supply to the fuel injection pump. This is because the Duramax does not come equipped with a electronic lift pump and relies on the gear pump on the fuel injection pump to draw fuel from the tank. You can confirm this with the GM Tech2 Scan tool by monitoring the desired and actual rail pressure when under heavy load, causing the desired rail pressure to increase near the top end of the limit. The top end limit is near 25,000 PSI. (Other scan tools give you some of this capability, but not all are able to see the desired and actual fuel pressure. This is important, so you can see if a large difference between the two set points exists.) This is best done real time with the passenger monitoring the desired and actual pressure while the driver puts the truck under load. Uphill, heavy throttle is best. If you see a difference greater than a couple hundred PSI between the desired and actual rail pressure then you need to perform the vacuum test on the supply side of the fuel system. This test is in the GM Service Manual and the test port is in the front passenger side of the engine bay. If you have not changed your fuel filter since then I would recommend that you start there prior to the testing, as a clogged fuel filter is a common cause of this issue.
If, after performing the vacuum test, you find an issue with the supply we would recommend that you consider upgrading your supply pump to one of the FASS units we offer. You can add a supply pump like the D-MAX 7001 or a complete Fuel Air Separation System like the Titanium Series. Please let us know if you have any additional questions. Thank you and have a great day.