Also consider what happens when this added cap is hit with enough word peaks, or a more consistent tone and actually reaches 2/3 or more of the charge voltage that the ALC line is applying to it.
If the ALC cannot respond to a fast peak, it will also be inhibited from reducing power in a fault condition such as an intermittent antenna connection. We should be adjusting resistor values to reduce the level of ALC rather than retarding its action with additional capacitance. The problem has nothing to do with reaction time and everything to do with the level at which ALC voltage begins to rise. Icom is notorious for having the most aggressive ALC of any major brand. I hate to say it but this is a bad modification.
