EGR valves
 

Would a faulty EGR valve trip a fault code? Or could there be problems that would only show up with a physical test and evaluation?

Another mechanic has suggested that an ongoing misfire in my A8 could be linked to such. Any thoughts?

Re: EGR valves
 

Sure it could. And a vacuum leak could also show, if the coated fabric membrane is ruptured.

Valve ports can become obstructed with tar or carbon exhaust gunk reducing function even if the valve itself is moving correctly. EGRs can jam solid at any position. EGR improperly happening at low revs generally means poor idle.