The 68000 can use every other cycle to access the bus, so that's 7 MB/s.
There's 7MB/S total to/from chipmem. The 68000 gets half of that at best, same with ROM and zorroII bus.
AGA machines have 4x the bandwidth (double clock rate and 32 bit access)
The memory bus isn't really double the clock rate, but there is 4x bandwidth (for the display, not for the CPU) because the chipset can do two reads in one turn with 64-bit aligned data.
Zorro III ...
It's 32-bit and 25MHz and should be able to go 80MB/S using burst mode although in practice it is more like 20MB/S as you say.
