It is true that so long as there are drivers, one is assured of some serial functionality... But Macs have always been funny when it comes to routing services with serial communications. For instance, why are most 3rd party adaptors incompatible with LocalTalk? It is still RS422, right? And then there are MIDI managers, such as OMS and CoreMIDI - they do not automatically assume that a serial port is legit for MIDI I/O. They typically require a MIDI driver. Some 3rd party drivers might work for other adaptors, while others might not.
A more pressing reason for my question was to find out what serial chipsets have built-in MacOS drivers. If it is the same as the RS232 ports in an Xserve then it should definitely work. But if it is some random serial card with no drivers, it would not be of any use at all.
I am not looking for raw data dumping speed, but rather precise timing. USB, like ethernet, often waits and packetizes blocks of data and sends them at "convenient" times. Whereas a real serial port can send and receive bits in real time. For tight MIDI timing it can be handy to avoid any additional levels of translation or abstraction which can be heard as it sends data. Keep in mind that USB was developed and pushed because it is cheaper, and better for dumping large files to and from appliances such as cameras and MP3 players. It was not meant for real time communication.
Excellent advice! The connector often is the most expensive part, and cheap manufacturers will frequently omit them while the functionality lies hidden on the board.