Temperatures aside, the gas pressure alone will rearrange the packing in any non-baffled muffler. As the supplier stated, the drone won't change because the anti-drone device is rigid unlike the packing material. Shinoo has made it clear that this is a street muffler, so sustained temps will be hugely lower especially since cooling air is present in spades (unlike on a dyno even with fans).
There, does that consolidate everything well? All parties seem to be making true statements but the context here is important. If you were happy with your non-rebuildable muffler before, then you won't have an issue here.
"Lots Of Trouble; Usually Serious"