You stated that this is a 2.5L TBI car and Pope may think this is a turbo car, 2 1/4" exhaust is not going to really do anything for a TBI car, which would also explain why he mentioned fuel pressure under load, on a turbo car rail pressure changes constantly as engine vacuum/pressure changes, on a TBI car pressure remains constant under all driving/load conditions, however...
Since your driveabilty issue is under load it is a good idea to check fuel pressure under load to be absolutely sure.
To check fuel pressure under load you would need to connect the fuel pressure gauge underhood at the TBI fuel inlet and run the gauge out from underhood and tape the gauge to the windshield and go for a ride (with a helper) and drive under the conditions where the problem is present and see if fuel pressure drops.
Cam Timing on the 2.2L/2.5L engines can be done by a Novice and is really not that difficult, gaining access is all nuts and bolts, aligning the marks properly make take a few tries to get it correct, we have all been there.
Also, the 2.2L/2.5L engines are non-interference engines so no worries there.
Checking Cam Timing is relatively simple.
1)Disconnect and isolate the - battery cable.
2)Remove the upper timing cover
(2 nuts on the valve cover and 1 bolt where the upper cover meets the lower cover)
(Inspect the belt for missing teeth while the cover is off)
3)Remove all 4 spark plugs.
4)Bring the piston on #1 cylinder to TDC on a compression stroke (distributor rotor pointing to #1 cylinder on the distributor cap.
5)Align the mark on the bellhousing timing marker with "0".
6)Check to see if the Cam sprocket is at the 12 o'clock position with the cylinder head.
This is the tricky part, the cylinder head is on an angle so the mark will not be 12-6. it will be close to 11-5.
(See the illustrations, if you are not sure take a pic)
Over the years I have seen strange things with cars, although you have been driving with no issues until recently if the Cat got worse it could be moving and blocking the exhaust when going up hill (Gravity), I have also seen foreign objects in fuel tanks that caused a problem when it moved to a certain position(we found a bee in the fuel tank of a Ford Truck that caused problems when turning).
The only way to find the cause of your issue is diagnose all of the systems and eliminate them as the cause, there are no shortcuts, if you say "it can't be that so I am just going to assume it is good and move on" you may have just bypassed your problem and then you will "be in so far you cannot see the forest for the trees", Been There, Done That!!!