This is interesting. I also got some of their 40den/70gsm fabric, as at the time I intended making two sleeping mats: one from 70den and one from the 40den.
Before starting I made test strips 2cm wide and welded them together at different temperatures. I am now using a converted soldering iron with temp control from 150C to 450C, and a shoe 11mm wide. I found it was virtually impossible to get a sweet spot where it was hot enough to weld but cool enough to not melt. And whenever it seemed to be a success, a hard pull would break the fabric at the juncture of the weld to the unwelded part.
This is exactly where Exped mats fail – frequently the baffles let go inside the mat. My last was UL with 20den fabric, and holding it up to a strong light let me peer inside and left me quite certain that the baffle failed at that juncture.
Extremtextil list the 40den as having a melting point of 163C. What are you using to weld with?