according to "Yamaha AICA Sound System Hardware Reference v0.8" You can change the clock rate of the arm chip through the interrupt/timer register 0x28a8 which is set to the desired clock speed - 1. The examples this document give are 00 = 1Mhz and 24 = 25Mhz. Using this you could theoretically clock this higher, to 16 Mhz, 32 or probably even 64 without causing any permanent damage. I assume that what is happening is that it defaults to 3Mhz (~2.8) because that is the minimum required by the AICA but it is probably possible to go much higher than that (probably up to 25Mhz as sega notes) safely.
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