Okay, after some helpful postings (Thanks Denny and Slim) and some outside research, I think I have this whole thing figured out. Please understand that I was not questioning your collective wisdom. I fully accept that you all know what you are talking about and I greatly respect you all for being so willing to help us "noobs" out. I am a mechanical engineer by trade though, and having to know "why" seems to be my constant curse and boon. So for those whose interest I spiked here goes my rendering of what all the fuss is about (Feel free to correct me if I misrepresent the subject somewhere!):
First off, there's a fairly good article on Wikipedia about backfiring in general and if you're so inclined, I'd recommend a quick read through.
Coming from a previous job as a combustion engineer for industrial burners (Natural Gas and various Mixed Gas experience), I was confusing myself a little on the subject. There are two conditions which can cause backfiring: Lean OR Rich.
As most of us know by experience to some degree, fuel can only combust under a certain set of conditions which are driven by things such as temperature, pressure, and perhaps most importantly, air/fuel mixture. A rich mixture has too much fuel and as a result, there is simply not enough air to have complete combustion, which results in unburnt fuel. This unburnt fuel is hot via the partial combustion and the introduction of oxygen causes a secondary burndown. This is what is typically experienced during throttle overrun, which is what I think my original issue with my Vulcan boils down to. Under hard throttling (accelerating or impressing your friends with your new-found noise maker), when you let off the throttle quickly, there is an excess of fuel due to your loaded-up carb which cannot be burnt completely in the cylinder. This causes an overrun backfire and in my case explains why I get a single loud "boom" followed by your typical runback popcorn. I think I can try to tune this a little bit with the idle/fuel screws and marbling, but it's never completely avoidable.
A lean mixture is a bird of a different color. The fuel mixture does combust, but the excessive air serves to retard combustion and as a result, unburnt hydrocarbons are formed. The combustion that does occur tends to be very localized and as a result causes a hotter firing cycle where combustion is taking place. This is part of the problem with running lean. My error in my previous thinking was not understanding why these burn with the addition of air from our air injection systems. Unburnt fuel, and particularly hydrocarbons are problematic in that they have ample temperature for ignition, but are simply not in the right air/fuel ratio for combustion...until they are via random mixing and molecule collisions. This secondary combustion with a lean mixture is unpredictable and as a result is difficult to control. This explains why the typical popping is common on the Vulcans. Kawasaki just wants to meet EPA standards (the government slap on the wrist hurts) and as such uses their version of Aspirated Air Injection to encourage the combustion of the hydrocarbons formed by the initial lean mixture partial combustion. Because the second burndown is unpredictable, Kawasaki uses excess air in conjunction with the "goat's belly" to promote mixing and attempt to control the popping. Those of us who remove the "goat" via de-goatting or after-market pipes notice the popping more so as the more controlled method of secondary burndown has been removed
Now, as for how our EPA "crapola" works. There is another good article on the basics on Wikipedia as well :
I highly recommend a cursory reading, but here is the important section:
"Aspirated air injection
Air injection can also be achieved by taking advantage of the negative pressure pulses in the exhaust system at engine idle. A sensitive reed valve assembly called the aspirator valve is placed in the air injection plumbing, which draws its air directly from the clean side of the air filter. During engine idle, brief but periodic negative pressure pulses in the exhaust system draw air through the aspirator valve and into the exhaust stream at the catalytic converter. This system, marketed as Pulse Air, was used by American Motors, Chrysler, and other manufacturers beginning in the 1970s. The aspirator provided advantages in cost, weight, packaging, and simplicity compared to the pump, but the aspirator functions only at idle and so admits significantly less air within a significantly narrower range of engine speeds compared to a pump. This system is still used on modern motorcycle engines, e.g. the Yamaha AIS (Air Injection System)."
So, our EPA equipment is a simple aspiration affair (there is no seperate air pump) and as such is only used at idle and low throttle conditions. Aside from being ineffective at high throttle inputs, I think the additional problem Kawasaki faced was that the Vulcans are tuned VERY lean at high end already, and this causes unwanted engine heat to begin with. The addition of more air at this point could serve to cause further problems. So for this reason, or some other I have yet to figure out, Kawasaki installed the air switch valve which is driven off the suction vent of the Vulcan's right-side carb. As throttle input increases the air valve is driven closed and prevents air from even reaching the reed valves, but opens as the throttle is released and the carb vacuum line returns to a lower vaccuum.
In summary, the popping we get on decellerating can come in two forms. At the first crack off of throttle, there is a chance of an overrun condition (harder throttling to sudden decel or coasting) followed by the much more common lean combustion popping. By eliminating the EPA injection we can remove the "fascilitating" factor presented by the air injection, but we also end up enriching our idle/fuel screws in order to encourage a more complete combustion in the cylinder to begin with, which in turn reduces the formation of those nasty hydrocarbons.
I hope this was helpful to others and that I have not led anyone too far from the beaten path. If I have gotten any of this wrong, please, PLEASE feel free to correct me. I am simply trying to learn how our bikes work and what our modifications are actually doing to the operation of the bike. I apologize for the length as well, but I tend to like to get it all out at once!
Thanks for all your help!