Cartridge bottom bracket left cup is too tight? Is it possible for the left-side of a cartridge-type bottom bracket to be too tight ? 
I followed the tightening torque instructions, and now the axle is not running as freely as when the cup is less tight.

Should I un-tighten it ? Or is it normal that there is some sort of "preload" on the bearings ?
It is never okay for there to be "preload" on any bearing set. It causes them to wear out prematurely.

Adjust it to where there's no play nor binding (or as close as you can get to it).

Thank you for being sensitive to the obscure mechanical needs of your bicycle.

The instructions are going to bias on the side of tightness, because most people will notice and complain about loose bearings but not tight bearings. And when the BB wears out after 20k miles, they won't know that it could have lasted 100k miles with proper adjustment.