When I last renewed the two centre pipes and back box on my 2.6, one had previously been replaced by a VX piece; the other was the original from new. The bolts came out of the original with no problem, but the bolts in the previously replaced section sheared off, leaving about 1/2" protruding.
This was the push I needed to buy a 'dogs 'dangle berries'' Rothenberger blow torch, which have great reviews, and it got the edge of the flange and sheared bolt not far from 'red'. With a bit of effort with some mole grips, and few cycles of heat, the sheared bolts came out.
It appeared that the bolts in the previous replacement may have been over tightened or 'stretched', and then sheared trying to remove them, whereas the original correctly torqued bolts came out without too much of an issue.
I replaced the bolts with original VX bolts - those on the replacement pieces I fitted were tightened sufficiently to support the exhaust and to provide a gas tight seal, but not excessively so; possibly most are probably unnecessarily over tightened on refitting which causes subsequent removal problems.
Just my experience with this particular car, and thoughts - the original fitting bolts don't need to be excessively tight; on other cars, I have drilled the sheared bolts out which is relatively difficult due to problems correctly aligning the drill itself with available space relative to the underside of the car. It was easier using heat.
Every time I am under the car, I usually just give the flange bolts a turn and then re-tighten, one at a time, just to keep them as free as possible to aid future removal.
Time will tell.