This is how I did it.
First, I subjected all the ribbons to tension test. The measurement was done quite roughly, by observing weigh scale. Anything close to 12 kg means that the seam isn't really weaker than the bulk of the ribbon.
Next, I subjected all the ribbons with the seam intact to a "belt test": I rubbed a 6 mm diameter metallic rod with the ribbons. When I say "very robust", I mean it didn't break after about 50 times the seam was against the rod; very fragile means it snapped after three or four rubs.
The precious results follow:
6. SplineTravel+SeamConcealment. snapped at 14 kg, close to the seam (~20mm to the right of the melted-like zone). Belt test: couldn't test it well because it broke near the seam, but it is very robust.
5. SeamConcealment alone. snapped at 9kg, at the false seam. Belt test impossible...
4. SplineTravel alone. snapped at 12.5 kg, away from the seam. In belt test it proved durable, but weaker than 6.
3. Slic3r's wipe while retracting. snapped at 11.5 kg, at the seam. Belt test impossible.
2. Slic3r with forced retract. snapped at 11.5 kg away from the seam. Belt test - very fragile.
1. Slic3r plain. snapped at 12.5 kg away from the seam. Belt test - slightly more durable than 2, but still quite fragile.