Never mind... partly answered with a second test, but I'll post here for reference.
It's an edge effect from one of these applications. regardless of which it is, increasing the camera FOV to 100° produced an almost imperceptible difference without feathering in PTGui, and now that the images overlap you can set the feathering in PTGui back to the default (middle) position for a perfect result.
PTGui puts a dark edge around the sun on tone-mapped TIF/JPG outputs, but the EXR output is fine.
Final PTGui project attached.