Do your benchmark results indicate any level of regression on Opus 4.6 or 4.5 since their first release?
But we did heavily resample Claude Opus 4.6 during the height of the degraded performance fiasco, and my takeaway is that API-based eval performance was... about the same. Claude Opus 4.6 was just never significantly better than 4.5.
But we don't really know if you're getting a different model when authenticated by OAUTH/subscription vs calling the API and paying usage prices. I definitely noticed performance issues recently, too, so I suspect it had more to do with subscription-only degradation and/or hastily shipped harness changes.
There's your major issue. That's well within the brutal quantization window.