What I think can help is make the fan to pull / extract the air use the original as it is stronger than Noctua (Arctic label pointing the PCB will make it to extract --- I checked their specs as I have Noctuas too) and add thermal pads to the bottom heatsinks 1mm @ 13+w/k will have the highest impact in it, those few changes will give you
650@2.5-2.6T stable on a ~120W on stock voltage.
Hm, why do you think the stock fan is stronger? When comparing the specs between the Noctua NF-A8 and the Arctic F8, I see the Noctua better in every discipline. Slightly higher RPM, better airflow and MUCH better static pressure. The latter of which makes it an even better candidate for blowing air into the unit (the original direction).
My bad i checked vs the Noctua R8 (the ones i have with me), I didnt saw you were using the A8, yes that is better to push but still it is weak, it will do better extracting thought but pushing these fans are quite weak.
having it to push and the original and/or other noctua pull will help, but i not sure how effective will be as it wont be in a close setup tunnel but i assume it will help to keep them cooler and accelerate the air causing it to cool a bit more.