When you say "HSV", do you mean feline herpes virus (FHV)?
Some people here on TCS use Lactoferrin as an immune booster for upper respiratory issues and FHV in conjuction with l-lysine. I only use l-lysine for my cats since they all gave FHV, and I have used Vetri-DMG in the past as an immune booster for my one cat during a major flare.
The suggested dosing for AHCC for cats of all sizes is 0.5 to 1 gram daily according to the free downloadable Patient's Guide to AHCC at their research website.
Just a thought that the dose could be divided in to two servings when mixed with wet food. A 500mg. capsule could be easily opened and used in this manner.
This info was found on page 68 of the Patient's Guide.