So, a few points that deserve elaborating on. First, the stat caps mentioned earlier include the stats already on the item. HQ battlecraft items are typical at the cap for stats on it, so materia is more useful for situations like adding crit to slots that don't have any, or adding vit to non-tank accessories.

Secondly, the speed it spiritbonds at depends on the level of the action (mob killed or item gathered/crafted) compared to the level of the item. Syncing for dungeons or fates does not change item level for this purpose, so even if you're level 22 in the dungeon, if your gear is 45 you won't get any SB. Should also be noted I mentioned speed: going after mobs 9 levels below the item level is abysmally slow, and higher levels things SB you faster (this is why folks SB in dungeons).

Speaking of speed, you can increase the speed a number of ways. HQ items SB faster than NQ, and attaching materia also makes it faster (good use for junk materia). There are also equipment that boost SB rate, but typically this is the only stat on it and takes up a slot you can use for more spiritbind items (the exception is the aetheryte ring, which has crafting stats and can be converted, albeit not into crafting materia). There is also a FC buff that increases SB rate, as well as spiritbind potions that increase the speed as well. Although, I do hear there's a cap, so don't expect penta-melded items, +8 spiritbind from gear, +5 from potion, and the tier II FC buff to SB stuff ludicrously fast.