[v8volvo]

Excess backpressure from a restricted exhaust is one possibility, but these leaks also may be just old gaskets failing. These gaskets are 35 year old parts well outside of their life expectancy and they are exposed to constant thermal cycling. Having to renew them at this point would not be surprising.

Probably best to start by just cleaning up the surfaces very well and trying some new gaskets (not silicone, need to get the correct type stainless steel or fiber composition exhaust gasket which as ngoma noted is the same as VW parts and widely available). With any luck this will stop the leaks and you won't have to pursue exhaust changes. Plugged mufflers are not unheard of but usually they only occur as a result of a very poorly tuned engine that is emitting an enormous amount of smoke, and/or one that is never driven hard enough to get exhaust temperatures and velocities to the necessary levels to keep the system clear. If your engine is running correctly and you drive it on the highway from time to time, it would be unusual for a muffler to become restricted with soot.

On the other hand, if gaskets keep on failing in these locations even after you replace them, then you will want to investigate possible exhaust restriction. Any loss of power? Restricted exhaust would make a noticeable impact on performance, especially in terms of much slower turbo spool time (bad turbo lag), weak high RPM performance, and excessive visible exhaust smoke.

The soot marking around the leaks is normal, however, as long as there is not a large amount of smoke out of the tailpipe when driving.