Q: Why the high overtier pipes have such low flow rates, or even slower than vanilla pipes?
A: There are two common causes:
Are your pipes quite empty? Are you having oversized pipes? Fluid level determines the pressure, and pressure determines the flow rate. If you have very few fluid in the pipe, the fluid will barely flow at all! Using narrower pipes will keep the fluid level and pressure high, so may actually have higher flow rate!
The second cause is very often a misunderstanding of Factorio fluid mechanics, such as improper order of pipes and pumps. Please read (again) the online documentation and guidelines listed in Note section on the main page.
Q: Why creating this mod? They are already existing mods improving the fluid flow...
A: Yes, there are several of them. But my prime motivation is to support the overtier machines (especially heat exchangers) which need enormous amount of fluid, but none of those mods can reaaly support to that scale... Moreover, the sizes would not fit well, so I have to create my own.
Q: You have mentioned others' mods in the Credit section. Why not just develop some extension mod from their code?
A: Yes, this was my first motivation when I found out "Fluid Must Flow" mod. It provides width2 pipe (called "duct" in his mod), so I will make an extension mod to provide width3, width4, etc, to act as pipes for my "Schall Machine Scaling" mod. But after some thorough testings, I realize it is using vanilla storage tank to mimic the duct. And using it will have faster pressure drop than expected (for some reasons), thus cannot reach my desired scale (of 4, 16, 64, etc.) of flow rates. Another mod "Flow Control" has similar problem.
Surprisingly, after some extensive experiments and understandings of Factorio fluid mechanics, I have found that I can just use the vanilla pipe type and it solved the problem... Therefore, I can just base on the vanilla pipe code and scale it like my previous "Schall Machine Scaling" mod. So finally, this mod comes out as the current "standalone" version.
Q: Why the size of high overtiers are not exactly scaling with the number of equivalent pipes, say pipe +2 being 9 (instead of 16 now) vanilla pipes?
A: Although it would be logical to scale like that, it would be less confusing to use the same scale as Schall Machine Scaling, since this mod is made to support that. Please read the corresponding FAQ.
Overtiers
Each higher tier is exactly 4 pipe/pump of its lower tier, in terms of flow rate, power consumption/production, or any other properties.
Total amount of raw materials to craft any overtier pipe/pump would be the same as the individual array of vanilla pipe/pump. Thus no real benefit or unfair advantage (cheat) in using this mod.
The two advantages are reducing total number of entities, and larger size so more beacons can be applied. These are intentional, since the prime motivation of this mod is to reduce the number of entities and improve UPS for megabases.
Overtier  Scale 

Vanilla  1 
+1  4 
+2  16 
+3  64 
+4  256 
+5  1024 
+6  4096 
+7  16384 
It is generally not the best to apply highest overtier to every fluid, because of the insufficient fluid level leads to low pressure, causing very slow flow.
Nuclear Plant Designs
When used with Schall Machine Scaling, it is possible to construction extremely powerful nuclear plants with minimal UPS hit!
Using high overtier nuclear reactors, and even higher overtier heat exchangers and steam turbines (also pipes and heat pipes), to keep number of entities low.
Using a solo nuclear reactor is very fuel inefficient. 2×2 is a good compromise between fuel efficiency and UPS. You may use 2×4 up to 2×8 to take more advantage from neighbouring bonus. But then heat exchangers and steam turbines will needs to be upgraded, to suppress the number of entities (thus fluid flow calculations).
Here are some example designs, which are all quite UPS friendly.
"Theoretical Rating" is the calculated maximum power output by theory.
"Actual Rating" is the displayed output on power pole interface. The value always appears lower because of display truncation, although it can also be caused by nonideal fluid flow.
Some blueprints are also using Schall Belt Configuration, for using loaders and belts of greater underground distance.

Nuclear Power +1 2×2
Theoretical Rating: 1.92 GW
Actual Rating: 1.9 GW
Blueprint string

Nuclear Power +2 2×2
Theoretical Rating: 7.68 GW
Actual Rating: 7.6 GW
Blueprint string

Nuclear Power +2 2×4
Theoretical Rating: 17.92 GW
Actual Rating: 17.9 GW
Blueprint string

Nuclear Power +2 2×8
Theoretical Rating: 38.4 GW
Actual Rating: 38.3 GW
Blueprint string

Nuclear Power +3 2×2
Theoretical Rating: 30.72 GW
Actual Rating: 30.7 GW
Blueprint string

Nuclear Power +4 2×2
Theoretical Rating: 122.88 GW
Actual Rating: 122 GW
Blueprint string

Nuclear Power +5 2×2
Theoretical Rating: 491.52 GW
Actual Rating: 491 GW
Blueprint string

Nuclear Power +6 2×2
Theoretical Rating: 1996.08 GW
Actual Rating: 1.9 TW
Blueprint string