IIRC 8.4s and 9s have a slightly different pull rate depending on if you've got strong or weak fingers
ftfy
The difference between 8.4 and 9 is about the same as change of cable pull of the latest Shimano levers relative to the old industry standard, but which way you mix and match them is down to personal preference. In theory, SRAM/Campag levers + CX8.4s should feel about the same as Shimano levers + CX9s, so the direction in which you move is different if you swap over.
ftfy
The difference between 8.4 and 9 is about the same as change of cable pull of the latest Shimano levers relative to the old industry standard, but which way you mix and match them is down to personal preference. In theory, SRAM/Campag levers + CX8.4s should feel about the same as Shimano levers + CX9s, so the direction in which you move is different if you swap over.