Mert is this you working on your own car or is this a customer car?
Anyway...seeing all that carbon in the cylinders doesn't surprise me. I theorized in another thread that the CBU starts in the cylinders. I know thats totally obvious so...
But seriously, a well running combustion chamber (IMHO) releases way less "stuff" that can get into these ports and (again IMHO) finish combusting in these channels in the hot environment. Older M5's with the more oil passing by the rings are even more susceptible.
Even if the exhaust isn't completing combustion in these channels...the exhaust system is a positive pressure environment. The gasses are being forced down the exhaust system under some pressure. This pressurized gas is always looking for a way out. Perhaps the addition of a one-way check valve in the SAP air line right where it enters the head would help. I'd bet that once the SAP turns off it's not airtight internally and this enables the exhaust air to travel thru the channels and exit out the SAP thru the SAP pumps fresh air intake. A one-way check valve at the entrance to the head would stop this if indeed it's happening as I suspect. Maybe mert could tell us if that SAP itself is loaded internally with CBU?
One other thing that I think would slow it down or stop it from happening is to have the secondary air pump running all the time the car is on. Not full power of course...maybe just at a few % of it's normal running speed, but not so much that the pre-cat O2's see excessive oxygen in the exhaust. This would create a positive pressure inside the small channels and keep the exhaust gasses at bay.
I'm not letting BMW off the hook for any design flaws here. I'm sure in hindsight they see where they went wrong. However I think this manually cleaning is treating the symptom not the cause. They should look for a solution for the SAPs shortcomings. My suggestions may not be the be all end all, but there's a decent CHANCE they could help. I think $8,000 with the threat of reoccurance shouldn't be their only response.
As far as us owners go in the short term the only thing I can suggest is RELIGIOUS maintenance and a shortening of any published BMW schedules for things like air filters, spark plugs, pre-cat O2 sensors etc. And monitoring the health of your thermostat. ANYTHING that affects combustion efficiency!