Although the exhausts were stainless steel it was not the high grade (last foreveer) stuff. Welded areas do corrode and both ends of the resonator box (little middle section) on my 2.8 were blowing slightly.
I hade this section cut out and my local indy made up a sleeved section to replace it. Cost me about £60 but he was doing other work so exact cost is not clear.
The result is a slight increae in "boom" at 1500-2000 rpm and a reduction in noise at 3000 rpm. At motorway speed there is very little engine/exhaust noise (a bonus for me), but the soundtrack is still the same for acceleration and over-run. Emissions seem unchanged as expected. Not sure if you would get similar resulrs with other engines.
This repair should last a few years but I expect that it will corrode again and require further work.