Gutless Fiero

My buddy has an '86.5 Fiero GT with the 5 speed manual. It has absolutely no power. It barely keeps up with my mom's Metro. His old 2.8L Fiero had much more pull than this one. I've checked out the ignition timing (spot on factory specs in both normal running and "timing adjustment" mode), vacuum (solid 17 inches, less than 1" fluctuation at any RPM, more stable the higher it is revved), compression (over 160 lbs for all cylinders, within 10 lbs each), Fuel pressure (at factory specs), and changed the TPS (tested with a dead spot, changing it cured a very laggy throttle), MAP (the hose to it holds vacuum), ECT, O2S (very worn out), and IAT. The EGR valve moves freely. The coil, plugs, wires, cap, and rotor are new, but all of the plugs are fuel fouling. There is a slight manifold exhaust leak on one bank.

The car is totally gutless and has a very violent exhaust backfire when shifting (the old car did the backfires but still had power). I'm at a loss as to what to check next? Perhaps a leaking injector? Ignition module? Could the manifold leak be fooling the OS2? Any suggestions?

My first guess is that the timing chain has skipped a tooth because its all worn-out. This means both intake and exhaust valve timing is late and will cause the symptoms as described.
He should not drive the car until he checks this out. If the timing chain skips another tooth (which it will eventually do) he could bend some valves.