I'm in contact with the Russian guy Sasha (the one who managed to flash the ECU first). There are some very big thread in Russian forums about the issue and his hypotheses is that the false knock is coming somewhere around the cams - he thinks the noise is from the cams when the AVCS angle is high (above 40 or so) - when I check my logs it goes to 50 degrees on wide open throttle between 2-3k rpm. I asked him what about if we change the OCVs and even the AVCS sprockets but he thinks this is not sure to fix the issue as the noise is coming (according to him) from cams or somewhere there - you know that with this angle the intake/exhaust valses are overlapped and definitely the sound changes. The common agreement among Russian people is that only ECU re-map can solve it once and for all. All the rest is just temporary (soldering capacitor to silence the knock sensor, official ECU update, etc). Maybe only the knock sensor removal (as somebody mentioned here) can also cure it but that is more or less dangerous as it can foul the ECU to pull timing advance which can cause real detonations and this is much worse compared with hesitations. Anyway the Russians just fixing it by ECU flash - Sasha made over 1000 cars in the region and his know-how is spread among other mappers, so there are a lot of people in Russia, Ukraine, Poland, etc. with such re-maps. However nobody there done any extensive verifications to confirm what is the root cause, as it's a lot cheaper just to map the ECU.
In my opinion on the high levels in FHI they know pretty well what is the problem, but that information is not available outside. Even the dealers does not know it except the service bulletins which in that case says to put 5W30 oil, clean the throttle and update ECU. If somebody have a way to find info from high levels of FHI, then we can eventually know the root cause, but I doubt that info is somehow secret.
What was the status with my car? Immediately after I bought it I got the issue (it has 115 kkm on the clock). Then I read here the info and start digging, then bought cable to do myself diagnostic and confirmed I have drop in the ignition timing. Then it fixed itself, and after 200 km made it again but not so bad, then disappeared and appeared again. The last time it was pretty bad with -10.5 knock correction. Anyway, I asked Sasha to tell me if I have all updates and he wanted to send him the ROM (it's possible to download it with USB Vagcom cable) and he told me I have one of the updates but not the last one. He thinks that if the first didn't solved it, then the second cannot help much but it's ok to try, so I installed the last update in Subaru dealer. The car runs fine now for about 1000 km without hesitations. Also after the update I had to reset the ECU as the learning tables for knock corrections are stored in the RAM, not the ROM which they updated. For now I will drive it to see when the hesitations come back (hopefully not soon). Sasha gave me good price for ECU remap (he can even send me ready to plug ecu) and when it start giving issues again I will probably move on with that. What he is basically do is to decrease the AVCS angles from up to 50 degrees BTDC to max 35 degrees (as you can see on the table on my previous post), then he turns off the EGR and secondary air system (if there is one, from MY2008 there is no such) as that is also giving issues. And I think he also do some re-map of fuel maps (probably to make it runs fine without EGR). I read also some feedback for his patch and generally it's good - most of the people report better and smoother torque in low revs, although some slight increase in consumption. Also they continue to confirm all is ok 3 years later. He provide modified vagcom cable which can be used for later upload of updated maps, so some complains can be fixed if it's not ok.
So for me after the tons of info I read, I think the only option we have for final solution is unofficial ECU re-map, since FHI do not care after the warranty finished. That is why they made ECU updates - just to keep the car without complains in the warranty period. Also I can speculate, the other reason is that this problem is not dangerous for the engine (since it retard timing, not advance it) and cannot lead to a lot of broken engines etc., which is worse for their reputation. Everything else is pretty expensive for them, and re-map by doing what Sasha do will probably make the engine ecology worse, which they cannot afford even more (in the context with VW scandal).