This is true. I had a built block GTX2867r turbo 2013 ST putting down 422whp with no issues back in 2014 on stock fuel system. I was well informed about the ST back in those days. The ECU in the ST was able to adjust the stock tune to minor changes such as exhaust or CAI to include adding 2 gal of e85 the ECU will advance timing on its own. Such an amazing car that now I own an RS for the AWD.Here you go in plain simple terms.
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In all seriousness. When I had my 14 ST, I remember lots of people running E30 mixtures in their cars for a little more pep. Quite a few were running it with out a tune with no problems. The rest of them running it got the Cobb tuner and I believe it came with a off the shelf tune for E30. I don't know 100% I never messed with it.
Now to the E30/e40 RS question? I currently have the NX2 Precision Turbo on e30 on my RS. Get the Tune+ ethenol analyzer. I wish I had that when I had the ST, I was guessing half the time back then. But now with the ethanol analyzer and using the Tune+ calculator I tend to hit the nail each time. If I dot hit E30 it's maybe e29 or e31. The tune can handle a change of -5 or +5 so I'm not worried...Im still learning about the RS as I've owned it less than a year so I'm not sure if e40 is pushing the stock fuel system to it's limits but I presume it does on a bigger turbo. On the stock turbo e40 may not yield more HP as the stock turbo is already maxed out on e30 or out of it's efficiency. Maybe others with more knowledge can chime in!