Those are some good reads. Here's what it boils down to...for me...after fiddling with things over the years.
First off, you need to keep the question at hand clear...and establish what you want to do really.
The primary effect that barrel length has on things is....the over all length of the gun. An MP5K can be made to shoot as hard as a M4....a VSR 10 can be made to shoot as soft as a springer pistol. So...re. barrel length...go with the length you need for your build, keeping the following in mind.
1. The barrel should remain centered in the outer barrel and not move for maximum accuracy...either by being a nice fit, using shims/tape/orings, etc...
2. That it be made to precise tolerances along it's entire length
3. Material selection should be made on performance vs. budget
So you can adjust your spring power/gears/motor/etc...in your build to deliver the speed/power that you want...and they should be matched to your barrel length for optimum performance and efficiency.
So here's a rephrase of your question (I think)...being a bit more specific:
Q. If I setup for the longer barrel length...what happens if I swap it for a shorter barrel without changing anything else?
A. Your FPS will drop. How much...it's hard to say, but say between a M4 barrel (363)and a shorter SIG552 (247) it might be 55-ish FPS. (I used to have a switch inner barrel setup that I could just drop a different hopup/inner into to use both for indoor and outdoor). As said...when you put a barrel in that is shorter than ideal, your bb will leave the barrel before the piston has finished it's forward stroke.
Q. If I setup for the shorter barrel length...what happens if I swap it for a longer barrel without changing anything else?
A. Theoretically (assuming the setup was super efficient)...your FPS will drop as the BB will still be in the barrel when the piston reaches it's most forward position and there'll be drag. In practice...the FPS will go up a bit until it gets to that drag point, because most setups are a little over powered anyways. What if you go full retard longer than the setup is made for (i.e. stick a 550mm barrel on a 3/4 port M4 setup)...you'll drop a lot of FPS. Might be 100+
Inner barrel diameter
What's the difference between a 6.03 vs. 6.01?
With a tighter bore...
+ velocity (how much? hard to tell but low of 5 and high of 15 usually)
- potential for jams (BB's are imperfect...dirt/dust/moisture is a factor)
+/- potential for more accuracy (usually the comparison is between old stock 6.08mm and 6.04-ish barrels)
It's really a balance between having enough space so there's a good cushion of air around the BB while it's carried down the bore...having an inner diameter matched such that the BB can stabilize...and accounting for the less than perfect reality of life.
6.04/6.03's...in a good setup...have always done the trick. To be honest though...I think more accuracy can be had by really consistent compression, a good hopup rubber and a matched barrel length...than sweating 0.02-0.04mm in the barrel diameter. There is absolutely no benefit to a tightbore if you're running a crappy stock hopup rubber with a leaky system.
Just my $0.02...don't sweat it too much. Just buy a good brand with a length to match your build...and match the cylinder/compression parts to suit that. Compare with some other brands on the barrel lengths...they're not all the same. Don't sweat 3-5mm difference...won't matter.