Whew! Lot to digest! Okay, let me try to answer each question individually.
For reference, since it wasn't written in the original post, the stock 17" tires have a load rating of 93. So why would the 235/40/18's in your original post (load rating 95) need a higher tire pressure?
When you go to a lower profile tire, you change the shape of the air chamber. In the case of going from 17 to 18, you are also making it smaller. Therefore, you generally have to squeeze a lot more air in there to carry the same load. I was quoting information from Toyo's Fitment Guide and I hate to contradict that information as it is researched quite thoroughly. In checking load and inflation tables, it would appear that you may be able to run lower air pressures than I originally stated. For example:
Size--Load Rating--Load @ 32 psi
However, I would tend to err on the side of caution and run a little higher than oe. Uneven wear would likely appear only on grossly overinflated tires, but my original point was to remind everyone the importance of maintaining proper air pressure.
Toyoguy, in practical purposes- what disadvantage is there to reducing the tire's rated load capicity? Is it a matter of safety, longevity, or something else?
Yes to all of those. You don't want to overload the tires as the heat buildup could cause catostrophic tire failure (worst case, depending on how overloaded the tire is). Heat will also cause the tire to wear quickly. You want the tire to be able to meet or exceed the highest stated GAWR (Gross Axle Weight Rating). I have a 6i 5-door. The front axles have a GAWR of 2370 lbs. This is the most amount of weight that the springs/shocks can handle. You can find the max load rating on the sidewall of the tire. The oe Michelins can carry 1433 lbs. @ 41 psi. Typically, we reduce the load rating by 10% as a safety margin. That gives you 1289 per tire x 2 = 2578 lbs per axle, which exceeds the GAWR of 2370. I run the 225/45R17 94W Proxes 4s which max out at 1477 lbs @ 50 psi.
I don't know what the 6 cylinder models state on the door placard. Perhaps someone could pm me. As you can guess, there are tires that are smaller than the oe tire that meet or exceed the GAWR, but then we strongly recommend that you do not go smaller than 2% of the original overall diameter to avoid negative effects to ABS and engine management systems, speedo, odo, etc.
One thing that worries me about my competition tires, which are deliberately small, is their very low load rating. Would that make them more prone to a blowout? While I race with them at 44psi front, 32psi rear, I still drive to and from events on them at "only" 36psi front.
I think you are running 215/45R16s? If they have a load index of 86, you will need to have at least 36 psi in them. At that inflation, they max out at 1168 lbs. That's less than the oe 16s carry at 32 psi and barely above the oe 17s. Those are considered the minimums required by Mazda. I hope you aren't carrying much other than yourself! You would really do better to have a larger size on there. They are 2 inches shorter than oe! Are you really benefitting that much from better gearing?
Is the load rating or recommended tire pressure based upon the volume of air it takes to fill a tire?
Yes and no. A tire's load rating depends on how much air the casing can hold. A standard load tire may max out at 44 psi, but actually does not carry any more load after 36 psi (there are slight differences with P-metric and Euro metric tires). A reinforced tire may carry up to 51 psi. If you can fit more air in the tire, it can carry more load.