1 minute of out combat + price increase to maybe 20/25MU + less damage over the AOE? or an even smaller cooldown. Something along those lines. But we have to keep in mind that this ability can make the cheap good at long range volks become lethal close range units.
They only become lethal at close range if you fail to dodge the grenade. All of your tests and your video showing how effective the grenades were is only because you stood right in the middle and let them hit you. You are given ample warning and time to dodge these grenades, and you yourself stated you were aware that they were throwing them but chose to ride them out. You talked earlier about how the models crawl around after the first explosion preventing you from dodging, but at that point you have already failed to dodge it.
It is fine for an ability to be powerful as long as it is easy to see and respond to, it effectively nullifies the abilities strength.