Assuming the computer is fast enough, is there any reason to keep the Gameplay opitons Body Decay Time, Maximum Number of Enemies, and Camera Distance to anything less than the maximum?

There isn't. It is even better to keep camera distance at maximum, so you see where you can port.

I do have the feeling that Maximum Number of Enemies actually influences the difficulty quite a bit...

Obviously. And combining maximum camera with minimum enemy might even regulate spawn-rate.
That's why you should not touch the original setting.

I am halfway in Eldritch playing with everything max out. I think I will continue like this, I tried out of curiosity to put Maximum Number of Enemies to the default value and the maps looked empty.