Does removing the check valve or using the 6.4's have any bad side effects? Lack of fuel pressure on start up? other?
Considering the fuel systems between the 6.4 and 6.0 are two COMPLETELY different systems, one could surmise there's a reason Ford designed it the way they did. The design of the check valve in the 6.0 was not designed to bog the truck down at WOT (I have stock bolts and have yet to have a WOT fuel starvation problem), but are designed to keep fuel to the injectors because of the design of the system, whereas the 6.4 operates similar to the D-max using a common rail system, one of the reason those two engines can produce gobs and gobs are power with the simplest of modifications.
Most members (not all, but quite a few) claim that they saw better fuel pressure and better throttle response but those same individuals installed their Ford Blue Spring kits concurrently with the 6.4 banjo bolts. Their pressure pressure increases and better throttle response was attributed strictly to the Blue Spring kit, not the banjo bolts.
Some people will also say things like "volume of fuel to the heads..blah blah blah." If you're concerned about volume, get a larger pump that will pump more fuel. 55psi is 55psi, regardless of the size of the line. What's important is whether or not a pump can handle flowing a larger volume at the required psi (I doubt the banjo bolt difference would cause a problem for a stock pump) but, if you want more fuel, get a bigger pump.
Again, those check valves are designed to ALWAYS KEEP FUEL AT THE INJECTORS. Removing those check valves removes the ability to keep the fuel where it needs to be.
Here's an article on how the fuel system works:
"Fuel System Operation
The 6.0L HEUI fuel system uses 4 main components, including a high-pressure oil pump (HPOP), a fuel lift pump, a Fuel Injection Control Module (FICM), and 8 hydraulic-electronic fuel injectors. For more on HEUI injection system operation, see our Powerstroke Diesel Injection article.
The HPOP pressurizes engine oil and delivers it to oil galleries mounted on top of the fuel injectors, under the valve covers. The HPOP oil pressure is controlled by an injection control pressure regulator (IPR), which is installed in the HPOP cover located at the rear of the engine valley, under the VGT. A signal is sent to the this IPR, forcing it to open or close, thereby regulating the volume and the pressure of the oil delivered to the oil galleries.
The fuel lift pump is mounted to the frame under the left-hand side of the cab. This pump delivers low-pressure fuel to the fuel galleries in the cylinder heads, through two fuel filters. The fuel galleries are cast into the iron cylinder heads, and are open to the fuel ports on the fuel injectors.
The Fuel Injection Control Module, or FICM, is an electronic module which provides a 48 Volt control signal to the HEUI fuel injectors, and works in conjunction with the Powertrain Control Module, or PCM, in operating the 6.0L fuel injection system.
The fuel injectors on the 6.0L have ports on top for high-pressure engine oil, as well as fuel ports located on the side, below the oil section of the fuel injector. A constant supply of fuel is allowed to enter the fuel injector, and high-pressure engine oil is allowed to enter the fuel injector through an electrically operated valve. The amount of fuel delivered by the fuel injector is determined by the fuel injector pulse width, which describes the amount of time the oil valve is open, as well as the injection control pressure, or the oil pressure which is controlled by the IPR. The high-pressure oil is used to pressurize fuel within the injector at a 7:1 ratio, which forces open the pintle at the tip of the fuel injector, allowing fuel to enter the combustion chamber.
During idling and lower engine RPM, multiple injection events may occur, allowing for more efficient, gradual and smoother combustion."
Notice, the highlighted area says nothing about volume of fuel from the banjo bolts. It's controlled by IPR, not banjo bolts holes. Until someone can provide sufficient data showing the volume as being increased, there's no convincing me it's worth the measly $10 and 1-2 hrs of my time. Just my $0.02