FX-32

Hi!
It's been practically a year since I'm having a weird issue with my idle since installing Long Tube Headers from FastIntentions (black ceramic coated, no wrap)

My issue is that AFR in both banks goes up and down, like from 10 to 20, in both banks at same time mostly.
I put a wideband AFR sensor were the Y-Pipe ends and it show the same as my Ecutek OBD info.

What I've done so far:
In october 2020 I put the headers, drove for a few days and then upgraded my 2008 ECU to a newer cloned ECU from a 2009+ 370z in order to get Ecutek Racerom features.

Then I noticed the bad idle. I installed:
- New OEM Denso O2 sensors, carefully extended, nothing changed
- New OEM Denso Spark Plugs and OEM coilpacks, nothing changed
- New fuel injectors, I put Injector Dynamics 1300x and tune, nothing changed
- CJM Fuel system with Walbro 450lph fuel pump, same issue as before
- Ported Upper and Lower intake manifolds with new gaskets, same issue
- Tuned with speed density instead of MAF, same issue
- Changed the rest of the exuast system, I had a Stillen catback and put a Motordyne resonated Y-Pipe with a stock almost new 370z exhaust to the end (resonators and muffler pieces). Same issue
- Downstream O2 sensors disabled by ECU and 4Bar MAP sensor. Nothing changed.
- New PCV valves, tried connected properly, then with catch cans, and vented. Same
- New cloned ECU to discard the ECU, it acted differently altough it had the same software and tune but nope, it still was idling weird.
- Installed a TopgunZ ACE kit, obviously didn't expect this to solve that issue but It was "scheduled".
- Lightweight flywheel and clutch (just to handle the extra torque)
- Oil pressure is correct, overkill yes but I change oil every 1000 miles so internally it's "like new"
- New exhaust manifold gaskets, studs and nuts. Tested for leaks by five different shops and all of them say "No Leaks Detected"
- Boost leak tested: No leaks (that means a healthy intake manifold, right? or not necessarily?)

I went this week to a very reputable dyno tuner to get a E85 tune, the car makes about 600whp (with stock y-pipe back), no knock at all (did a 4th gear pull with Ecutek datalogging and evaluated the logs). At this dyno tuner they left it idling at about 1000 - 1100 RPM and told me that because of my Long Tubes that the AFR acts like that.

I noticed some variations in AFR while driving and got back to a reputable remote Ecutek Master Tuner who showed me, evaluating the logs together, that this tune has some issues with fueling, it's mostly lean, and I can see that as well while driving on the wideband gauge I put. He suggested that I have an exhaust leak in the headers since day one, that's why I went to so many exhaust shops, to test for leaks and everyone say that I don't have leaks (I wonder how exhaustive that test is, if it's only by hearing and smelling or with proper smoke tool).

The only thing left to do for me is to put a Tomei Headers and test pipes, but not only is more money to throw parts and see what happens, but also a lot of work (I do all the work on my workplace's parking lot because I don't have a garage and I live in an apartment), and the long tube headers is one of the hardest jobs and I don't want to get in there again, and I'm running the risk that it couldn't solve the issue and end up with an inferior header installed and having to do all the work again to put it back as it is now.

So, to recap, My remote tuner says it's an exhaust leak, and the dyno tuner says it's because of the long tube headers, says no leaks at all (no boost leaks and no exhaust leaks)
I expected this dyno tuner to solve this but he says it's just annoying but not an issue, while driving I also see my AFR Gauge fluctuating a lot.

I was thinking in trying UpRev with stock 2008 ECU but since I have boosted and E85 I was told by many people that it's inferior and might need multiple maps depending on ethanol concentration to select them manually (Ecutek is automatic)

Any ideas or pointers what could I try next?