My car runs rough with the stock tune and 3 bar. Ran it idling for about 15 seconds when I did a reflash. No CELs, but it was idling rough. Wait to actually install the 3 bar until you are ready to load the 3 bar tune. Its a 3 minute job, so I guess if you wanted to practice, you could.
The T Stat was a much bigger deal for me.