My thoughts:
-If you can swing it, I'd buy the ZZP downpipe. Besides the U bend area, the stock double wall pipe up top is much smaller inside than it appears externally.

-If thats too much, just go to an exhaust shop and have them straight pipe the u bend area. May be hit or miss on places thatll touch it as some shops get funny about doing cat related stuff and this section contains the rear O2 sensor for cat monitoring. No reason they can't, just makes some shops skittish.

-Id not delete the cat. The performance difference is negligible on a stock car and all it does is add fuel to the fire for the continued spread of emissions crack downs / sales of aftermarket performance parts. Its just a matter of time before they stop the sales of 'off road' pipes because too many people use it to circumvent emissions equipment. There are situations where running a cat can be difficult, but a stock monte is not it.

-I wouldnt buy a full cat back. No idea what they sell for these days, but they used to run $1k-$1500. Thats likely almost half the value of the car for near zero performance gain. I get mandrel bent 3" tubing sounds like it would flow so much better than what you've got, but its not the primary restriction on a stock car. If youre missing mufflers, I'd just head down to the local exhaust shop and have them weld up some new ones.

-I wonder if some of the disparity between results of resonator deletes stems from cars coming with different styles of resonators from the factory? I've had them off of a few W Body cars and have seen 2 distinct types - one was the perforated cheese grater style that hung into the exhaust the whole length, the other was literally straight piped throguh with the exception of one small tube that hung into the stream. Sadly I don't have pics readily available, nor do I recall which type came from which of my cars, but I was surprised to see how different they were despite being used on the same platform.