Here is the thread:
In a way your right calling it EU3 could be misleading rather I should have called it EU2-CAT vs. EU2-Cat Preparation. EU2-CAT vs. EU3 seems to be looking for exactly the same things but they do indeed have different software version numbers. I'll break down how I even went looking for that thread and how the info lead me to getting my car to pass emissions.
I flashed the EU2 tune which is 7545671. I left UIF writes enabled, my old software version was 7533159. If you see at the bottom of this picture it says CAT. I drove on this tune for a couple days and no longer had a CEL on in my gauge cluster. Now I may be wrong but from what I read in Europe the secondary O2s are not only used for monitoring catalytic converter performance but to also further dial in fuel. Anyway I scanned the car to see if I was ready for my e-test. One of my secondary O2s is bad and when I scanned the car even with no CEL light on I had four codes. Two codes pulled up threads on UK BMW forums so they were European specific. I had a cat efficiency code and a code for the sensor that is bad being stuck rich, and a couple other related codes. Obviously I couldn't go for a test and the car was still looking for a cat. I was happy with the tune as I described above but I didn't feel it did much to make the car any quicker, I was fine with that I just did this to pass my e-test.
Now a couple people said you must pull the SAP relay and fuse before flashing to have a true EU2 tune but after finding the thread I linked above it said you just have to reset adaptations. I didn't take a picture but the last line of the picture above changed to say CAT-preparation after pulling the fuse and relay and resetting adaptations. Now what the thread fails to mention is pulling fuse 4 disables all of your O2 sensors so the car would be running open-loop all the time meaning it's referencing a VE table and using your coolant temp, air intake temp, maf, TPS etc to calculate your AFR. I thought this can't be very efficient but lets see what happens. This VE table would have been designed for countries that had cars shipped with this manifold:
You can even see the plugs where the O2 sensor would go meaning it's true open-loop. There are still some adaptations that will happen over time but not based on the o2 sensor readings. This is probably why the car feels quicker because it has to be referencing a VE table that is designed for the catless manifold which isn't very different than ebay headers.
If you flash a EU2 tune and still have cats or headers with non foulers to trick the ECU you are still using the primary o2 sensors. You won't have any CELS and the car will feel a bit better and you'd even pass emissions but your tune would still be geared toward a car with cats.
I only found this out because I never even bothered with the non fouler trick and had a bad O2 sensor which logged the codes that made me dig a little further into this.
So in your case if you have headers and a EU2 tune go in to INPA and see if it says CAT or CAT-preparation. If it says CAT and you pull the SAP relay, fuse 4 and reset adaptations you'll feel a noticeable difference in power.
I've noticed no adverse effect to my fuel economy, I've put a few hundred kms on this tune and it seems to be doing a bit better even.
Some of this information is missing from that thread and I plan to post a response based on my experience.
I've built, wired up and tuned two E30s on megasquirt,one was turbo putting down 330 WHP and another was just a ETA/325i hybrid but I became pretty familiar with how you can make the cars run such as Alpha-N mode and I believe that is how my DME is running now.
The difference is it's running Alpha-N mode with no errors. If you are running US spec software and one of your o2 sensors fail it will throw a code and run open loop and maybe richen things up and pulls some timing and you'll notice
your fuel economy will suffer. With the EU2 in CAT-preparation mode I believe it's more than just failsafe it's optimized as some cars are delivered with this tune such as in Poland/Russia etc.
I'm guessing BWM shipped some cars to countries where the cats were added at the dealer level before delivery and rather than require a reprogram they would just need to reset adaptations?
Anyway without a dyno I could just be talking out of my ass but if you're familiar with INPA why not try it and see how your car feels after? If it already says CAT-preparation then you're already good to go.