In a situation with 1 uC and many serial ports I would recommend closely coupled UARTs. You want the UART wasting it's time with the slow RS-232 or MIDI transaction, not the micro.

It is much faster to execute a single write like a mov SBUF,A or to external memory than it is to bit-bang an SPI port. I have used the SPI UARTs by Maxim and they are fine. USE A MICROCONTROLLER WITH BUILT-IN SPI. The Philips one may also be suitable. It's somewhat unpleasant to have a large mess of SPI devices, but it will be much faster than bit-banging a bunch of SPI ports on different pins, Failing that, find a UART (or bunch of UARTs) that you can map to XDATA space so you can address them quickly.

GB