Thank you for your reply.
I don't think the car request more fuel Under WOT than the fuel system can deliver because :
1) It's a simple Flexfuel tune so I have exactly the same hp/torque as stock. No performance related, and the High fuel pump can handle injecting 15% more for the E85. It's litterally a tune that takes information from lambda sensor to recalculate the proper quantity of fuel that needs to be injected (depending on my E85 content, thus a flexfuel tune).
2) The car does the same thing even when I run regular gas (SP98 in France, which is 98 octane number). And with regular gas, the fuel pump should be able to deliver without any issue (ps : when I run 98 number WITH my E85 Flexfuel tune. I don't remember if the car did this with the OEM tune, because as soon as I got the car, I did the tune like 600km after so I didn't push the car really hard).
I will still try to go back to my OEM tune and run the car with 98 octane (regular gas) just to check if the issue is related to my E85 tune, but for these reasons, I think the problem is somewhere else, don't you think so ?
To answer the rest of your message, why would I reduce airflow (lower my boost level) as the car is 100% stock performance wise ? Couldn't the problem be related to a defective MAF ? Or a loose spark plug or anything else ?
Thank you so much for your help. I really love this car and I want to fix it not to sell it