After some driving around for two seasons I definitely concluded that the HPX MAF doesn't automatically correct for air temperature.

Every time I would tune the car, the WOT pulls would have AFRs off as IATs change between pulls. This became evident when there were abrupt ambient temperature changes.
My readings started to get really lean with high LTFTs as the summer came, to the point where after some heat soak and IATs in 60C the O2 corrections would be in 20s, load at minimum of 15% (while normally it is 17-18) and so on, indicative of under reported MAF reading.

I looked at HPX pin out and sure enough... it has IAT sensor pin out which implies it wants ECU to take care of temperature corrections. As a reminder, stock Hitachi/Bosch auto-compensate internally for IATs @ MAF and give real air flow.

Fear not though. Tune can take care of it, PUKANS table can propely adjust MAF for IATs so I will be test driving this feature. For starters, I found a temperature vs density table which gave me an idea of what I am looking at multiplier wise... and by calculating some stuff, this is exactly the kind of phenomena I am observing % wise on my deviations dependent on ambient temps:



This is PUKANS I arrived doing some interpolations between values (I used 20C as base as this is what temp my tune behaves best at) from the table etc and will report at some later date how it goes:

Wow, I had been noticing the same behavior (rising LTFTs recently as it got warm). I just switched gas stations so I had attributed it to a different ethanol % at the new gas station (I run e85).  

I think it should be the inverse of what you posted. The FR says T0/tans where T0=273 K
 
Theoretical result is first, calculated with a base of 21.75 C in parenthesis

-48: 273/225 = 1.21 (1.30)
-28.5: 273/245 = 1.11 (1.19)
-8.25: 273/265 = 1.03 (1.11)
12: 273/285 = 0.96 (1.03)
21.75: 273/295 = 0.93 (1.00)
31.5: 273/305 = 0.90 (0.96)
42: 273/315 = 0.87 (0.93)
72: 273/345 = 0.79 (0.85)

This makes more sense to me, as the air gets hotter and less dense, you will have a lower mass flow across the MAF at the same air velocity (which translates to voltage)

Edit: I didn't use a base of 20 C

I'm not sure what you mean by that, when I looked at the function here's how I interpreted it. I may have just overlooked something though

With any given air velocity (MAF Voltage) and a fixed diameter MAF housing (and therefore a fixed volumetric flow);
Hotter air (lower density) = lower mass flow (puan & fpuk < 1)
Colder air (higher density) = higher mass flow (puan & fpuk >1)


edit: I just re-read your first post, and I guess our trims are actually going in opposite directions. My car has been running richer lately (-12% LTFT this morning), when I said rising LTFT earlier I meant getting farther away from zero.

Since SY_TURBO is set to TRUE in our tunes thus post "KFPU" module, SY_TURBO = false chooses fpuk variable, but SY_TURBO = TRUE  chooses "1.0" so PUKANS has no influence here... I wonder if we could have assembly magician flip that around or always use PUKANS / fpuk?

Unless you have an aftermarket MAF, your problem lies elsewhere as your MAF is already correcting airflow for temperature.

Julex, have you been able to log temperature from your MAF?

Nope. I think my regulator was/is a major part of the problem though. It is definitely dropping pressure as it is heating up. It is an AEM unit with aluminum housing and steel spring, you know where I am going with this eh?

I observed that if the engine bay gets heat soaked, the pressure drops. If I adjust the pressure on hot FPR, the cold FPR gives me then +20 psi or so, way off scale.

I have a feeling that the aluminum upper part of FPR is heating up and expanding while steel spring inside, which is in contact with metal part of diaphragm and gets it cooling from there, is not heating up and expanding as much... Plus aluminum expands 2x the rate of steel too. The result is that the hotter the FPR gets, the less effective tension on the spring as distance it covers is increasing.

I also had large orifice in the FPR which was too big, even slightest tension change when adjusting the pressure resulted in large pressure change... this fact and that temperature drift were resulting in pressure swings with temps.

I since then replaced large orifice with a medium one and pressures are very stable. I still see quite a bit of LTFTs drift from cold -> hot engine bay but it is a fraction of what it used to be.

So, I might have blamed the MAF for what is really FPR's doings. Lesson is that adjustable fprs like AEM should not be installed in engine bay or at least adjusted when hot and run at low to middle pressures.

I am actually gonna put in hood vents this week (look for updates on this on audizine s4 / audizine allroad / quattroworl allroad), basically I will do what honda s2k are doing to their cars (and it does look good imho except that I will replicate vent shape and rough placement of what the audi below has):

There under the yellow hose:

IMO most of the effect is from direct heat transfer from the head->intake manifold->rail

I have no direct evidence though, just a feeling.