another idea which I dont know if it would work or not, is isolate the clock line to the microprocessor and interface it with an ICS511 clock multiplier. this will double your clock without affecting the rest of the system. Your still bottlenecked by the bus regardless. 

This doubling shouldnt affect asynchronous bus transfers because everything is handled by wait states and DTACK. But the synchronous bus may have problems. I dont know.