Lies! Lies, and more lies, I tell you!
Any section of the loop going from 3/8" to 1/2" will improve the flow rate, albeit not in a significantly measurable way. Most blocks perform better at higher flow rates, until you hit that point of diminishing returns, but unless one is running an outrageously expensive pump, "you ain't there yet!", and if you do have one of those outrageously expensive pumps, then you probably know what you're doing anyways...
As I've stated before, 1.5 gpm is the limit at which flow rate caused a significant restriction in the tubing, at 3/8" ID. Most people achieve 1.0 gpm so either way: IT DOESN'T MATTER! but if you get up to 1.5 gpm, then be serious about your tubing, and use 1/2" ID. Besides, 1/2" fittings are so much easier to find at the local hardware store, than 3/8".
Ok, I've said my peace...
