Prefixed instruction seems having only one M1 : Z80 doc show that a prefixed instruction take 4T more time.
Is IO_ACK itself a separated instruction ? I think nonot, it's more about a hack of a current instruction, adding two autowait and making its business during this inserted times.
IO_ACK offset into INT (interrupt) should not implicated by WAIT_n generator, and it seems that a WAIT_n during T2 is ignored because of autowait already inserted at this moment... for synchronizing an IO_ACK, I have normaly to insert WAIT_n during T2+2. No way, instruction itself is synchronized, so IO_ACK is synchronized also, you don't have to insert WAIT_n during T2+2.
So you slow down instructions following a slowing down instruction table, slowing it the less you can, and then IO_ACK comes or not, and then you synchronize next M1 putting WAIT_n during T2 modulo 4. IO_ACK two autowaits are not prolongated.
ralferro explain also that stretching instruction timing depends of memory used or not by instruction. I know that Amstrad schematics does not use the MEM_WR wire. So it could be hard to deduce if they added 1 or more WAIT_n for certain instructions. But I'm more about 1 WAIT_n inserted each time (it's more easy to hard implements), and the modulo 4 synchro, let's see results of my current experiment (comparing time instruction of Z80 and plustest.dsk testbench, deducing diff table of "slowing down instructions") wip.
=== Test of a real Zilog 80 ===