Yes, I've also hit it with Opus 5.5, but on way fewer projects. My long standing projects that I marked as failed/to-soon with 4.8 have entered the grey area of slow-progress-with-handholding like smaller projects did in the past.

To my experience, it's a simple complexity curve where the first bits go fast and the speed goes down very fast once you reach a certain level of complexity. Size doesn't matter as much as complexity: with the right foundations, it keeps zooming through it. Now I do wonder how much my inexperience in the complicated project is a brake on progress vs. it just being a hard to tackle problem.

My sense is the limit is more tied to the problem than the code base size. I work in some very large code bases, but it's not an issue because the nature of the problem (think what a standard SaaS would need).

On the other hand, I have a TS only project core is sub-20k LoC (NES harness), where it is struggling (for now, see other comments)