Sorry for the necromancy, but it seemed appropriate as I regularly face off against Mozza Knights, and I don't generally lurk in the inquisitional sections.
Grenades turn GK from pretty killy to mop the floor with almost anything, you are totally correct in your analysis. It also helps that my two armies are Tyranids and Elfdar (psychic power!)
The reason why they are so powerful (especially Rad and Psych) is because of their inherent rule manipulation. Rad Grenades allow you to effectively instakill MC, or almost any other high T model with your HammerHand Hammers, (or any other albeit rare S10 in Knights.) This alone is enough to neuter most squads, as Toughness is really a huge determining factor in assaults. (Ask any Tyranid player if they like Enfeeble.) Also it really isn't that expensive to buy.
Psychotroke grenades are actually one of the few wargear items I think are broken. (Ruleswise there are few things I think are stupid, this is one,) not only because of it's super powerful shutting down effects on the fast armies (who just happen to be full of psykers,) but also because of the offensive and defensive use they have. Something this powerful should have been on assault only. Actually I believe they are the only grenades in the game that work when you assault and when you get assaulted.
Buy them every game you can. They are annoying and will decimate everything because of it. Throw a force activation or banner in there and watch everything get pulped by a squad of marines. (Almost all the heavy Melee HQ are also either Daemon or Psykers too.. How coincidental.)