gotta admit mate that if you searched this forum long and hard, you wont find anyone else this has happened to (cats and turbo knackered from oil) and that in itself should ring alarm bells.
We all chat with each other and loads of other audi folk, some experts some just drivers, but across forum world, info spreads fast and wide. A car fault cant hide these days, it goes round like wildfire and we all know about it. Thousands of people, all driving the same cars and hitting the same faults. Think about it, that pool of information is deep and unless your "expert" is a trained motor engineer (audi / vw preferably) with a conscience then you need to take what they say with a pinch of salt mate. We have nothing to gain from mis-advising you, but they do.
Usually when some "mechanic" or "expert" states something we know isnt true to a new member, or that we have never heard of, the member will usually return much later with all the "expert"-suggested bits changed and the fault still there, and we usually end up at our original conclusions, ive seen it many many times on here.
get a second opinion in person from a qualified motor engineer or 2 round your way, and if they agree with yer man, then fine, i'll eat my hat
Hope you get to the bottom of it though mate.
Also think of this.....exhaust pressure is powerful, an engine is a fantastic compressor, pushing 400psi cylinder pressures at times, then that hot expanding gas is pumped down the pipe and out......what happens if you block that pipe, with say, a clogged cat, then drive it on the road at 3000 rpm?, the same huge pressures would be developed in the blocked section of exhaust, and something would have soon gone pop in the motor or the primary pipework, no engine can sustain an exhaust blockage for long without bursting the pipe somewhere else or being so sluggish you would never get to any speed,
but yours drives ok otherwise, and just wont rev over 3k???
c'mon think about it.