Another batch of PCBs arrived a couple of days ago and I'm slowly working my way through assembling them.
The one thing I've very happy with is that by replacing the relatively slow (in terms of access speed) EPROM that contains the Monitor firmware with an ATMEL EEPROM, I've managed to run the system including backplane and a couple of expansion cards up to 8MHz. The EEPROM is rated at 120ns which is around 8.3MHz.
The CPU is spec'd to around 14MHz so that would require memory and EEPROM with an access speed of around 70ns. This maybe pushing it for an EEPROM so some research on what's available is required.
However, running at 8MHz my SAD (Segment Address Decoder) board, and new 8-LED display board all worked fine.