So here's a quick account of my adventures in Illinois emissions testing, hopefully it will help others in the same situation.
This summer I installed a 42dd TBE, resonated with high-flow cat. Stage 2 APR tune (testpipe). I foolishly went to get tested with the testpipe file, and failed due to sensors not being ready.
I got re-flashed (at GRD) to the non-testpipe file, and was getting CEL for two codes - P2196,P0137. These are for sensor 1 - sensor signal stuck rich. And, sensor 2 - Sensor circuit low voltage.
I installed some off-brand o2 sensor spacer, and drove around for a while and got a CEL in under 50 miles, same codes. 42dd o2 sensor spacers were on backorder, so I'm like well fuck - now what. I waited and finally got one, and when I went to install it, noticed I had a MASSIVE exhaust leak post-cat/pre-sensor. When I fixed this and installed the 42dd spacer, I passed the test, no CEL after 50 miles, and the car is driving better than ever, including gas mileage.
I can't say whether or not the 42dd spacer is the miracle cure, as I did fix that huge leak at the same time, but I passed regardless. I should note that after you clear your codes, you should drive around for 40-50 miles to make sure the sensors go back to 'ready' state. They are not ready immediately after clearing codes, so you'd fail the test if you just clear the codes and get tested right away.
Hopefully having all this info in one place will be helpful for someone, I know I found it to be a pretty stressful experience.
TL;DR version - 42dd high-flow cat TBE, 42dd o2 sensor spacer, stage 2 (APR) non-testpipe file = PASS.