The problem with all of these 'tweaks' and I'll include tuning in that too. Is that you need to be very careful about what your criteria for success is and how you're going to objectively measure it.
I'm unclear about what your criteria for success is - you've said that you're going to modify one gun and make a comparison with another (very similar unmodified gun) but what exactly are you comparing between the two?
There's a real problem here, and that is that you're very likely to compare how one gun 'feels' to shoot against how the other one 'feels' to shoot and use that as your basis for deciding whether your tweak has been a success. How a gun feels is a very subjective thing, and if you're used to shooting PCP's a springer will feel bloody horrible to shoot because it bounces around and kicks you in the shoulder.
Now you could argue (and I think you are arguing this) that surely if a gun kicks less and feels better to shoot, it should be more accurate. However that doesn't necessarily follow with springers I have found.
For me, the one and only criteria I'm interested in is - can I hit more stuff with this. I don't care what it feels like, I don't care how hard it is to cock, I don't care how consistent it is over a chrono, I don't care if I can balance a dozen pellets on the turret without them falling off when I take a shot and I don't care it it twangs like a church bell when I shoot it. I only care about - real world accuracy, because that's what they give out trophies for.
Accuracy is a difficult thing to to test on a springer, to test it objectively you need to test it over a long period of time (at least a couple of competitions and a couple of tins of pellets). and you need to be so familiar with your springer and your technique that you're able to identify when you've made a bad shot and when you've made a good shot.
If you don't have a reasonable and repeatable level of accuracy before you start accuracy testing, it's impossible to gauge whether a gun post tweaking/tuning has improved or not.
The consequence of not having a clearly defined criteria for success and not having a repeatable and objective method of measuring your success is that you'll inevitably end up coming up with other 'theories' about how you can further improve things and you'll justify these theories to yourself based on the flawed results of previous tests.
What starts out as a 30 minute job of putting some rubber in a stock ends up being years of wasted effort on a series of wild goose chases.