First, is it a 6.2 or 6.5?
And secondly, it is natural to want more out of an engine. But remember you are not dealing with an engine built to the hilt. They are great working engines that will last a LONG time with good care (mine has nearly 30 years on the odometer with never needing to crack open the engine, only an injector pump and GPs over the years on a NA Burb that did its share of towing), and will give you great mileage as well.
You can push it harder, and it's encouraged because we all do it, but there is a limit to what a stock engine can so. As for the pump question, a mechanical pump that physically fits on it but which pumps out more is ok as long as you can provide the air to burn it, or the injectors to handle it. Plus your fuel is all passing through a pre-cup, so there is a limit.