Just thought I'd give an update on this that might help someone else:

I also had a P0420 code showing up on my '01 Monte. I had a break in the flex pipe which I was in the midst of fixing and a mechanic told me "Oh btw your catalytic converter appears to be plugged also". So I ordered an Evan/Fischer flex pipe/catalytic converter off Ebay and had it installed.

Much to my surprise it has shifted as smooth as butter since leaving the garage yesterday. I did some reading a while ago where apparently a bad cat can cause the P1811 long shift code to manifest when there actually isn't really a problem with the transmission.

Just thought I'd throw out there that I can attest that a bad cat does appear to affect the shifting of the transmission. Maybe it will help someone else?