Your supposition is correct, a kill in a party (even just you and your chocobo) binds roughly 40% less per kill than killing the same target outside a party, but if you can kill twice the number of targets while in a party than you can kill by yourself in the same time period you will come out ahead.

The data in both your links is out of date also. Patch 2.35 change how spiritbonding works. Spiritbonding works best when you kill targets with levels roughly 8 levels higher than the item's ilevel. The monsters in level 50 Dungeons have ilevels roughly equal to the level of the equipment that is gained in at dungeon.