I am not sure it is with all model #3's either. Maybe I got lucky with mine or maybe it has to do with meat/probe placement, but I haven't experienced the getting to temp issue that you unfortunately have been having.
My first smoke was a 7.76lb Boston Butt (relatively small amount of meat considering the size of the #3) that I put in the smoker at midnight with ambient temp overnight around 0 and during the next day in the single digits. My temps ranged between 185 and 265 during the entire smoke.
A couple of days later I smoked 3 large St Louis cut rib slabs and did have to bump my dial to 240 to get what I guessed was an average of 225. I placed two slabs on the top rack and one in the middle on a rack below it. I placed my probe about 2-3 inches from the wall on the single slab shelf. I didn't document the swings, but based on my memory I am guessing they were in the 185-215 range when I had it set at 225, and 195 to 235 after I bumped the dial to 240.
I am going to give the #3 a real test this Thursday (2/27) where I will be smoking another Boston Butt (this one 7.25lbs) with projected ambient temperatures of -18 degrees overnight and a high on Friday of -7. So, this will be a big test of the insulation on the SI #3 and if it will have any affect on my max temps. I'll create a new thread for this in the #3 section.
I can understand your frustrations and it sounds like you really know your Q. Hopefully with a few more smokes you will get a better idea of if you have a problem or not.
I knew about the WSM's, but wasn't really sure how to use one. So, your accolades around this unit prompted me to watch some videos. For me, while I don't doubt that they put out some awesome flavor, having to mess with that large amount of charcoal and the hassle of monitoring it is not something I would be interested in at all. The "Lazy Q" as Tony calls it is what I was looking for.
I have always been a propane grill guy too and my brother's incessant razzing me about how much better his Weber was prompted me to buy a Weber Little Smokey to use when camping. I like the Charcoal taste, but I have put out some very flavorful meat off of my Weber propane grill as well.
I have never had a charcoal smoker (mine have always been propane/electric), but have enjoyed several meats from friend's charcoal smokers, and for me I think get a better taste of the wood accents/flavors in the Smokin-It verses the Charcoal smokers where my taste buds tell me that it is more of a combination of charcoal "and" wood taste. That's just me though, and maybe others will disagree.
I am really hoping that you get better results going forward. If not, I am sure Steve will find a way to make it right for you. It seems with your analytical background, you may end up having to get an Auber to truly be satisfied with the Smokin-It. But time will tell. We're all here to help in any way that we can.