This is just a pedantic technical correction since we agree on all the practical implications, but nothing involving FSMs grows nearly as fast as Busy Beaver. The relevant complexity class for the hardest problems concerning FSMs, such as determining whether two regular expressions represent the same language, is the class of EXPSPACE-complete problems. This is as opposed to R for decidable problems, and RE and co-RE for semidecidable problems like the halting problem. Those classes are way, WAY bigger than EXPSPACE.