the low output has nothing to do with exhaust, everything to do with cylinder not firing.

no spark or weak spark or no fuel will be the cause, almost certain.

a previous car of mine (z28 camaro) had the same problem you describe twice.

1st time was a shorted spark plug wire (shorted against the heat shield designed to protect from the exhaust manifold)

2nd time was a defect in the distributor

I would check for spark at the plug; in the old days we'd do this by pulling each plug while running (thick rubber gloves or 1000v pliers is good; it was bad enough when cars used 15-20kv spark but new cars with 30-45kv really sucks). when you pull a plug and the engine doesn't change it's tune you've found the misfiring cylinder. Not sure how a new engine deals with this method of diagnostics but it should work.

If not the spark, most likely a fuel injection problem. injector not getting signal or gummed up or just plain broken.

ps when i read the title i thought you were coming back to say you missed your X5 and came back for company.