Fuel screw limits the cc's injected. Some engines, depending on design, will alter the timing if the volume is adjusted. Smoke screw also limits fuel cc's but it is boost psi modulated usually.
When ran right on the edge, timing can mean the difference between a powerhouse and burned valves or torched pistons. You can see that in the unlimited pulling tractors. And that difference could be as little as half a degree. A 190hp 6bt is nowhere near the edge. So you won't find that changing it 2 degrees will make an engine go from 7 mpg to 12. That's something it won't do unless your engine is way out of wack to begin with, and if that's the case, it will be running pretty poorly, and is likely a lot more then 2 degrees out.
I've said since day one that cam timing isn't critical on medium and heavy duty diesel engines. That's not just from me, it came from one of my old professors at college(who's forgotten more about diesels then most people could ever know). I've heard huge claims made by some companies by changing to their camshaft, But I've never seen a back to back dyno test from one cam to the other, just seat of the pants claims that they make. They always seem to add bigger injectors, different turbos, or some other item which completely skews the results because those other items will increase hp by themselves. Think about it like this. If the trucking industry could simply swap a camshaft and make just 1 mpg improvement, with the millions of miles traveled in a year you don't think they would do that?
What an injector all does depends on the type. The one in the 6bt is essentially a check valve with a nozzle, like what ramses said. They don't control timing, and they don't control volume. In a t444e, they not only do the injection of fuel, they also create the fuel pressure, control the timing, and the amount injected.
That's why there is such a discrepency between injector costs. You can buy an entire set of injectors for an old 7.3 idi for what one would cost for the 6.0.
Ramses, fuel isn't compressable like air, it has a fixed volume. So the fuel won't be pumped by the piston faster then what the injector can release it. Unless it's all out of wack and you're using way too small of injectors, whereby the nozzles become a restriction, and you'll likely blow off the tip.