Objective-C has indeed some impressive features, but also some inefficiencies and it got a bit out of favour over the years. Coming from the Wirth school I also understand the virtue and challenge of writing a compiler in his own language, but in practice it is often rather an obstacle than a benefit, and if a language is useful, there should never be a lack of representative projects. I thus write my own compilers always in a conservative C++98 subset or even in C so bootstrapping is never an issue and there is no second compiler necessary.
Is your blewit, which sounds yet like another big code base, also AI generated/assisted? If I understand you correctly, you indeed have the engineering expertise to write compilers and to understand its architecture and complexities; so the project is not "vibe coded", but rather "augmented engineering"; I didn't manage to have a detailed look at the large code base yet, but in that case I would expect that it rather looks like a human engineered code base than the often strange looking architecture and code which I already have seen in Claude projects where the AI is just left alone for a few weeks.
A performance within 1.1 to 1.8 on average of optimized Clang is a very good start for such a project.
As I understand, the AI has implemented the frontend and lowering after your spec, which is amazing and demonstrates that it is able to work not only in and for the languages it was trained on. How was the AI able to demonstrate that your spec has been completely and correctly implemented? Is the language and IR spec somewhere on the web? What formalism did you use for the spec?
I well remember GEM and also wrote software for Atari ST many years ago ;-)