Aside from a couple times writing to the slow WD750 took forever (like 9x longer than writing to the 850), everything else from gameplay, frame rate hitches, and loading times were basically a wash between the WD750, WD850 and PS5's internal SSD.
In other words, the WD750 3.2gb/s SSD is just as good as the internal SSD.
If you look closely at the gameplay triple comparison, there are times the WD drives frame rates are a bit better than PS5's for a fraction of a second.
For all you PC gamers who'd chime in once in a while in an SSD thread saying your PC's slower SSD could probably do R&C's time warping tunnels. You're right. A PC 3.2gb/s SSD did it fine.