I have a Raspberry Pi 3 Model B currently controlling and powering a strip of WS2812B LED's. A separate 5V 15A power supply powers the Pi through pins 2 and 6, while the LED's are powered off of pins 4 and 9, receiving data from pin 18.
The only issue I seem to be having with this setup is that on the first 146 of 300 NeoPixels (since I don't plan to use the whole strip, however I have not cut the strip anywhere yet), noticeable brightness drop is evident throughout depending on the color/brightness/etc. By my calculations, each NeoPixel uses up to 60mA and the Pi up to 2A, meaning a 15A power supply should be more than enough to power it. Alas, that seems not to be the case, and my question is why.
I should also mention that the same results are achieved when running power through a breadboard and sending power to the Pi and to the LED's such that the LED's don't directly rely on the Pi for power.
(My knowledge of electrical engineering is very limited, and so comments on whether or not powering the LED's directly from the Pi is discouraged would be appreciated as well.)