You need to set the Photoshop origin. It is common to use guides (I believe guides are what you are calling gridlines) and then set the origin to the guides. That is because Photoshop sometimes forgets the origin location. To fix it you can just set the origin to where the guides are.

Misaki the bone length is not typically used at runtime except to draw debug lines for the bones

Close, but bone length is used for IK and path constraints. Still it makes no difference to performance.

once There should be no difference in performance depending on the appearance of bones because the bone length is not typically used at runtime except to draw debug lines for the bones.

If you are interested in the details of each skeleton data format(JSON/Binary), you can find the documentation here:
http://esotericsoftware.com/spine-json-format
http://esotericsoftware.com/spine-binary-format

Basic information on performance is available on the Metrics view page of the Spine User Guide, so I highly recommend reading: http://esotericsoftware.com/spine-metrics