techknight said:

they probably use PLL clock multiplier ICs on the upgrade board. since technically later 68K and PPC chips are asynchronous bus transfer, it doesnt matter how slow the bus is, you just have alot of wait states and poor performance. 

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I think that by the time of the 500 MHz G3 NuBus upgrade, the logic board bus speed did not affect the ultimate G3 CPU speed, or affected it very little.   That's what I remember, in a not very reliable way.   TK's information would tend to support that memory.