According to a PDF published by pipe manufacturer Orion (part of Watts company), there is a measurable difference in pressure loss in sweeps vs regular 90 degree elbows. The pressure loss numbers vary by different pipe diameters. See the attached PDF, or here is a download link:
https://media.wattswater.com/Orion-HP-FrictionLoss.pdf
I agree that the gray PVC pipe/conduit is (by Code) meant for electrical conduit. It is not pressure rated. A local building / code inspector may not approve substituting gray conduit for Schedule 40 pipe. Where I am the gray conduit recently was roughly twice the price of same-size Schedule 40 PVC pipe.
If one wanted to use sweep elbows with PVC Schedule 40 water pipe, they are available. For instance, 1.5" PVC sweep:
Spears 406-015S - 1-1/2" PVC Sch. 40 90° Sweep Elbow - PVC Sch. 40 90° Sweep Elbow
www.supplyhouse.com
There is a price to pay for that sweep though, as a regular 1.5" elbow is less than half the price of the sweep:
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