I too have a tendency to put my trust in the mfr's engineering expertise but recent events across the spectrum have clearly demonstrated that may not be wise. They are all under far too much pressure to conform to increasingly tight emissions and fuel economy metrics while still delivering an expected level of driving performance. The last time anything similar happened (the decade of the seventies) was a perfect example: in that era we had Cadillac's V8-6-4, the GM V8 diesel, highly restrictive first gen catalytic convertors, under-hood vacuum tube plumbing nightmares and imports that, while improving vastly on reliability issues of the domestics could barely keep pace with a 'double-nickel' national speed limit, among others. This era (so far) it's Ford's EccoStar, Hyundai's gas mileage claims, VW's TDI emissions claims, seizing turbos and carbon building, degrading performance GDI's across the industry.
Hardly any mystery why currently pleased owners are curious and skittish about what revelations lie next as many of the latest problems can be resolved, often by doing something the engineers just missed telling us about. Or got over-ruled by marketing who're still reaching for the 100,000 mile maintenance-free automobile nirvana. (At which point it's a throw-away, btw. So what about those who'd like to keep it longer??)
Which brings it back yet again to OP's question: has anyone seen an opened up a high mileage SA-g engine to get a feel for carbon buildup?