OK, I am trying to keep my car on the road, but I am having trouble making sense of what I am being told by the mechanic. The engine has had half rebuilt and the timing chain issue resolved.
Now, after having some fuel rich/lean condition, the engine is running rough under load. When I watch the fuel trim values, they spike when under load (not in neutral or under constant speed) and I get misfires. I have replaced the plugs, coils, air filter, and MAF which made the situation a little better (smoother but same issue). The shop said they did a leak test and got 80 PSI on 6 of the 8 cylinders, but I only get misfires on Bank 2 (numbers 5-8).
This started after I used a fuel treatment (techron) to try and address the rich/lean condition.
Could it be O2? Can I narrow down the compression issue? The shop just told me to get a new engine (ha), but there are many causes of compression loss. Any suggestions?
First, why was the engine rebuilt? You mention the timing chain. Was there some sort of issue with the timing chain breaking?
If so… I fear you’re going to be chasing problems for a long time. Usually when the timing belt (or chain) breaks, the advice to get a new engine is the best advice.
This engine is done. 80 psi is about 60 to 90 psi too low. If a squirt of oil in the cylinder raises that 80 psi reading it is definitely the rings. If it doesn’t a valve job might bring it back… but that is a ton of money to do on this engine.
Your mechanic is correct. New engine time.
Edit: These compression values might be OK if you were at about 12,000 feet above sea level. 
You beat me to it!
If I was in the OP’s situation, I wouldn’t spend the big bucks necessary to overhaul the engine.
I’m sure some of these Audi V8 vehicles (didn’t say which model) have been driving faster then the driver had talent, so you might have a chance at a low milage used engine… If in the USA look at car-part.com, put your VIN in and start looking…
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No brainer, that compression number is too low for the engine to ever run correctly. It may be possible, if you are lucky, to retain this engine by just replacing the piston rings. Suggest to discuss that idea w/your shop pro. I expect they’ll say that while it might indeed be feasible, it might not make economic sense, less expensive just to bite the bullet and install a known-good replacement engine. Consider replacing engine oil & filter a little more frequently after new engine is installed.
Maybe a lawnmower story would help. Many years ago my 2-stroke lawnmower was getting very difficult to start. I’d have to pull on that rope over and over before it would pop and start running. I decided to replace the piston rings on a flyer. Fairly easy job on a lawnmower engine. Sure enough, with new piston rings, engine would pop & start & purr like a kitten, every time, the first pull.