Meanwhile, I think for the first time in 4 years or so, I will need to open up my boxer as it suddenly seems to develop an unbalance in temperature above 6500~7000 RPM (in flight).
Both cylinders rn basically near perfectly equal all the way from idle to 6500 where temperatures seem to slowly start to run apart, and at 7K there is about 10 deg C difference, at WOT (8200 in the air) that difference gets up to 20 degrees or more.
That in itself is not all that shocking, but it never did that before. What is more interesting, is that the cold cylinder basically does not follow the throttle above 6500 RPM (with todays temperatures it ran about 100~105 deg C and remains at that temperature all the way to WOT, while the other cylinder increases temperature with throttle.

That is a bit of a weird symptom, if I had to guess, there is either a blockage in the intake port or the intake runner.
Parkplugs are pretty clean, spark gap maybe slightly larger than 0,5 mm but not excessive, and as far as visible through the plugholes, the piston tops look fairly clean (I did not see any deposits on the piston tops).
But maybe it is something as silly as a leaking O-ring.
What is even weirder, is that I do not see deterioration of performance. It still does its leisurely 7500 on the ground and 8200 in the air, fuel consumption is normal, oil residue is normal. Starting is normal, basically everything is normal, only that load-related change in temperature difference.

Very curious IF I find anything, and if so, what it will turn out to be.

For what it is worth: the runhour counter passed 54 hours today (which means about 69 actual hours testbench included) and the last hour or so the engine was ran fairly hard because I was "calibrating" the fuel ROB indication (throttle dependent timer that counts back to zero, faster if the throttle is advanced, slower if the throttle is reduced, manual reset so its value remains even when the transmitter is switched of).
That works fairly well, I must say. It is not perfect, but good enough that I dare trust it. Done about 10 tests with either constant throttle (a few flightsboth high and low) and also with constantly varying throttle, and every time when the counter came to zero there was somewhere around 15~25% of fuel remaining.
Today alone I went through about a quart of fuel, and that is quite a bit...