Not to jump in on a discussion, but if you have a Gen IV PCM the best way to have fast accurate changes in ethanol content is to change from a virtual sensor to a physical sensor. The sensor mounts up by the rail so there is never a doubt what is going into the injector. It’s one PCM pin and a coil pack jump over harness. I think the kit from DSX is like $350. If your boosted, it’s a crucial piece of data.
Edit: you need HP Tuners (or a tuner) to switch the method of measurement. Might be able to do it with TDS, but I never tried.
This is what the V community does to run E85. The GM Flex Fuel Sensor (~$100) attaches inline, right before the fuel rails. I highly recommend DSX, they make quality products in the USA with a lifetime warranty! I have installed a number of these systems. Just remember, E85 being 108 octane will require a lot more pump volume, typically 20%-30%, which is also why I needed the DSX Auxiliary Pump Kit on my V. Without the Aux Kit, fuel starvation would destroy my motor. CTS-V2's have a unique situation in that for 50/50 weight balance, the fuel tank straddles the IRS. One must remove the entire rear cradle/suspension to access the tank top which is EXTREMELY time consuming and difficult, even with a lift/professional tools.
Two Options for V2's:
1) Cut a "access port" through the sheet-metal, under the rear seats and above the gas tank's pumping assembly to change internal pumps. Now they sell Fuel Pump Access Hatch "kit" that includes everything needed; template for cutting, rubber seals, screws and a custom fabbed steel hatch for the sealing the new opening. The idea of me cutting my car up was out of the question.
2) The DSX Auxiliary Pump Kit (and their copycats) requiring a single tap in the tank. Everything else is plug and play, controlled by the MAP sensor, accessible and proven extremely reliable. This aux kit requires NO CUTTING, just a single tap into the gas tank. After a ECM tune, the DSX Aux pump will engage if the FPCM demands 8+PSI and turns off at 6PSI. The kit is rated to 1000hp in the CTS-V2 application I believe.