Yeah. And I'm not sure that even that would give you the worst case as far as the cache is concerned. Of course I don't know how these implementations work, but it seems plausible that code that directly uses memory could run faster than code that encounters a cache miss beforehand (or contention, if you're using multiple cores). Moreover there's also the instruction cache, and I'm not sure if you can disable caching for that in a meaningful way?

For soft real time, I don't see a problem. But for hard real time, it seems a bit scary.