2008 Hard transmission shifts

2008 Impala, 3.9l engine, 155,000 miles. 20,000 on reman transmission.
Noticeable hard shift between 3 and 4 that showed up a couple months ago. Seemed to start out only when trans was cold and would get better after an hour of driving. Seems to do it all the time now hot or cold. Took it back to transmission guy who installed reman unit May 2013 and 20,000 miles ago. He says there is nothing wrong with the transmission and the computer is calling for higher line pressure than needed for the shift. He also says the shifts are coming on late. Suggested I have a driveablilty problem with the engine like plugged catalytic converter or bad sensor on engine. Car is reading and storing no codes at this time. Trans guy sugggested may just have to run car until something throws a code. Checked temp on catalytic conveter and it was putting out 700 degrees plus on the back end. Looking for any ideas what you would look at or have seen in past to solve the shift issue. Thanks!

BTW what trans do you have. also the newer Impala's have a transmission control module whereas my 2003 has a 4T65E which is fully governed by the PCM. I had a simular issue, sometimes when I started out i would get a shift right in the middle of 1st gear, still stayed in first till it was time to really shift into second. I soon after had my downstream Ox sensor go bad, after replacing the issue went away. I kept the old sensor may just need cleaning. Try disconnecting your battery overnight , will reset your PCM. Take all the underhood connectors apart clean with electrical contact cleaner while the battery is disconnected. Any cleaner used on internal sensors or intake has to say sensor safe on the can. Could be your MAF. My sister's Nissan's MAF needed cleaning , never would start out in first gear till I cleaned the MAF. On my 4T65E there is a known issue with the PCS (pressure control solenoid), it lets fluid leak past and so the line pressure drops, the PCM see's this and commands a higher pressure to protect the clutch pack which causes harsh shifts. What happens when you get the harsh shift if you turn the key off and start again does it go away?