Wow - a lot to go over here.
I agree, the nominal dimensions of pipe and tubing make no sense. As as been stated, for PVC pipe, the outside diameters are standardized schedule 40 and 80 fittings and pipe can be used with each other. This means that schedule 80 pipe has a smaller inside diameter for a given size, and therefore more resistance (see below.)
Also, as multiple people have stated, schedule 80 is completly unnecessary for our applications. I was recently at a water park in Wisconsin Dells - the water slides had PVC pipe pumping thousands of gallons per hour up 4+ stories, and it was schedule 40 pipe. If you need the pressure rating or ‘beefiness’ of schedule 80, you are doing something wrong. In general, schedule 80 is just a good way to spend more money for less flow. Note that this doesn’t necessarily apply to fittings/valves. I know many people who get schedule 80 valves because of the higher quality.
Regarding resistance and flow, there are many factors that go into the resistance of your plumbing. The single most important is the pipe diameter. The mathematics can get pretty complicated, but for simple calculations, the resistance to flow is inversely proportional to the forth power of the radius (1/r^4) This means that all things being equal, the resistance of a length of 1” pipe is 8 times that of a 2” pipe. If you use the actual ID of schedule 40 pipe, moving from 1” to ¾” pipe increases the resistance by about 2.7 times.
Fittings are generally the second largest source of resistance to flow. Reducing fittings will abosultely increase flow, so smart plumbing design that reduces fittings pays off. From what I can tell, two 45º fittings are about equal to one 90º in terms of resistance, so you don’t necessarily gain by 45’s to replace a 90. (As a side note, total resistance is additive, so it can pay off to use larger pipe, even if you are limited to a 3/4” bulkhead at the tank.)
Vinyl can reduce resistance to flow if it reduced the number of fittings. The two drawbacks to vinyl tubing are the barbed fittings which can be quite restrictive and the fact that it allows algae to grow on the inside which can signficantly affect flow over time. Another potential drawback to vinyl tubing is the fact that barbed fittings are more likely to fail than a glued PVC fitting. metal pipe camps the most secure, but are not very practical in saltwater applications.
For reference, I looked up the inside diameters fo PVC pipe. You can do the relevant calculations yourself, but for ¾” pipe, schedule 80 gives you 50% more resistance to flow.
Schedule 40
¾” 0.804”
1” 1.209”
1¼” 1.36”
Schedule 80
¾” 0.722”
1” 0.936”
1¼” 1.255”
. I'm not a fan of the all orange piping. 
