The bots I use needed the specialbot = true in the relevant `RegisterHam`s, which required moving from amxmodx 1.8.2 to 1.8.3, but after that the damage seems to work.
...is there a reason for HE grenades being excluded from the damage modification
edit - I started messing around with it, and it seems that the plugin applies the damage based on the active weapon - works fine for hitscan, but it's kinda funny how pulling out a boosted gun makes the grenade cause the boosted damage, but dropping that weapon makes it do regular damage. It seems that it needs some sort of special exception for inflictor entity of class "grenade". I will try to make this work.
edit2 - attached a version where grenades seem to work. I only focused on damage, no idea whether any other feature works, probably not. Needs amxmodx 1.8.3:
http://www.amxmodx.org/snapshots.php
I wanted the updated version for hamsandwich weird bot compatibility thing, but you can edit the RegisterHam lines that have the true argument to make it work with 1.8.2 - remove the trues (the `, true` part, the comma has to go, too) and it *should* be good to go on 1.8.2.