adding information regarding expected timing in the LPM_ADD_SUB section of the user guide

As you state, there are just too many possibilities of device and options to put something in a user guide. That's the whole point of the timing analyzer in Quartus. You can add something to a design and then quickly get this information for the device you are targeting with the options you select.

If something like this was in a user guide, how are you seeing its use for a designer?

And as for pipelining, well, in theory, you can never have too many pipeline stages if your goal is a fast running design if you are ok in the rest of your design with the extra cycles of latency. This is the whole idea behind hyperflex in Stratix 10 and Agilex. And fast forward compile with those devices will tell you how many pipeline stages would be most optimal.