I have + 5 spirit bonding however the gear that I am spirit bonding has no discernable extra benefit. Of course this was over a sample size of 25 crafts each...small sample size but figure was worth noting. Maybe others noticed different? I guess only real way to tell is to do some ridiculous long craft sessions. Craft only the same item as a control and always craft items starting at zero percent.

Maybe it helps spirit bonding lower level gear?