No, I'm sure lots of things have changed. More of the code would be using C99 semantics now, and I have no idea how GCC would have improved or regressed since then. It's possible that it's trading improved runtime performance on ever-more complex hardware for longer compile times.
I see that gnu89 is the default, to avoid compatibility problems with older code, but does that mean there is no code in the kernel which requires gnu99? I saw some work was done to make the whole kernel gnu11 compatible as well.
I recall, back in the long long ago, running through `make menuconfig` and disabling what I didn't need to get a smaller kernel + shorter build-times.