The per-thread caches are not strictly necessary, but you don't want threads to block because they can't stream in geometry to memory. You want some upper bound on the amount of memory each thread needs for geometry (each thread could try to ray intersect a different mesh). Dividing geometry into fixed sized chunks gives you this upper bound (max N chunks per thread).

Ray re-ordering is indeed a nightmare, but sorting very large ray batches (millions of rays) into coherent ray streams is less of a hassle, and it should enable coherent geometry access (which amortizes the cost of those reads, not sure to what extent).