"Ray detail region" and "Ray detail region padding" have nothing to do with GI. (Well, that's not entirely true, but doesn't affect GI the way you think it does.) These settings are mostly for ray traced shadows and ray traced reflections.
For crops that need to be stitched together, just set the region to "Detail in Camera", which will make each crop use the same detail region as the whole image (i.e. what the camera sees). If you do this, you don't need to adjust the ray detail region padding. If you used a padding of 0 for your original render, you should use 0 on your crops too. If you needed to use "Detail in Crop" for some reason, e.g. if you were rendering a very, very big image where the "in camera" setting is not possible, then you might want to try some padding of the crop region. Otherwise you should be OK with 0 (or whatever you had for your original image).
Lastly, ray detail region padding is not in pixels, it is a multiplier of whatever the current region is (on each side, in addition to the non-padded region).
Some more info from the changes log:
"Two new render settings on the Advanced tab which control the region in which ray traced polygons are fully subdivided. Outside of this region the ray traced polygons are only coarsely subdivided. Previously this region was the frustum seen by the rendered image or crop region, but now you can also choose to have no detail or to have detail everywhere within the camera frustum regardless of crop settings. "Ray detail region padding" can be used to enlarge (or shrink) the region. A value of 0 means no padding. A value of 1 adds a border to each side of the frustum which is equivalent to the width or height of the image or crop region, which makes the frustum 3 times as wide and 3 times as tall."
Matt