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My question: What does shimming actually do?

Background: I have a 2005 America. It came to me with Staintune exhaust installed on her. After reading a few posts on here about this particular exhaust it seems like the instructions that came with the exhaust told the installer to remove the snorkel, put 135 main jets in the carbs, and block off the AI ports. It looks like all of this has been done to my bike prior to my purchasing it in July of 2020. The previous owner was not a DIY'er so I'm sure he had a shop/dealership do the work. Before finding out the instructions told the installer to do this I had taken the carbs off and cleaned them, and I thought it was weird that there were no shims but 135 mains, which seemed really rich to me. I'll add that there is NO backfire... none... not even during decel.

I tried the jet calculator:

That brought up more questions than I had answers...
1. What does "Airbox Drilled" mean?
2. There are three options under "Snorkel Status"; 1) "All In", 2) "Top Removed", and 3) "All Removed"

Based on the calculator, and what I think is my current condition, my mains should be at 131.5 with no shim, and 129.5 if I put one shim. What I'm observing is I think the bike feels weak trying to maintain a specific speed... If I try to maintain 40 mph, it feels like I have to either let the speed s-l-o-w-l-y drop or give it just a touch more gas and accelerate. I say "think", because I don't have a lot of experience with this bike and it rides VERY differently from my other bike. (for one thing, it seems like the throttle moves much further on the Triumph as it does on the Raider... 40 degrees might be full throttle on the Raider but the Triumph needs 60 degrees of twist to hit full throttle... but I digress). Anyways, my other questions are:
3. Should I put a shim under each needle to improve mid-throttle power? There is no lag of any kind (a.k.a. throttle response is perfect) if I go from 'maintain throttle position' to WOT so I'm not sure a shim will do anything?
4. Should I try 130 main jets? I'm wondering if she is running much richer than she needs to be and could benefit, performance wise, from a downsize in jets.

Pilot jets are 42 (stock).

Perhaps the only solution is to find an identical America and see if I can take it for a spin, see if my bike is low on power do to too rich, or if it's fine.
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