Like m-f-f said, the exhaust mods you made require re-chipping or other measures, still I sense that with such a dramatic loss of power there is something else amiss.
Check the tank tank vent by blowing with your breath in to the end of the hose. Are you running vapor recovery (charcoal canister)? Any vacuum in the tank? Unplug the Lambda sensor and try no jumpers. Never had to resort to alt jumper with the 1100, Motronic 2.2 is not the same as 1150.
Since this is a WOT problem could be fuel or TPS.
Clean the injector screen (any tank liner flaking?).
Did you clean the ports in pump plate when you changed the pump? usually will have lots of detonation or pinking about 4-5k under load, hard accel if the mixture is too lean especially on CA winter mix fuel.
FTI if the TPS goes bad then symptoms will be much like a clogged fuel filter with serious loss of power because Motronic has faulted to limp home. Read the fault codes.
Finally, remove the intake manifolds and check them -very- carefully. Built in o-ring on the head side may be gone. Clamp may have been over-tightened. OZ guys have reported success in "repairing" them with o2 sensor safe high temp RTV.
If you pull the plugs don't forget to clean the head airways top, front, back thoroughly before de-plugging. Avoid frying Motronic by not cranking the engine unless a new plug is fully grounded.