Our house rule is if it's a headshot, then the target makes a Toughness roll (difficulty GM's discretion depending on weapon) or get knocked out (or stunned at GM's discretion depending on weapon and/or head armor). The roll is required even if damage doesn't penetrate.
Gives additional incentive for players to have their characters wear helmets and provide more variation by adding additional protective qualities to some helmets. I.E. The standard Cadian-pattern helmet has "excellent" protective properties against shrapnel, but not concussive effects. The OLD helmets did, but the Mechanicus doesn't know how to make those anymore. You know, that "Dark Age of Technology" thing.
Did you also incorporate the extras from Deathwatch, maybe? I kind of like how Space Marines can get the Command bonus for their SMsanshelmetitus, an otherwise often simply fatal disease many ranking Space Marines seem to suffer from. While I can understand wanting to get your players to more often wear their helmets, it might just be better to persuade them to keep their heads down, and use cover. Many of the ranking dudes in the TT IG, even the ones who don't have names, often seem to doff their helmets, in favor of keeping their senses sharp, and inspiring the troops around them. If I flip through stuff, Sergeants are helmetless, CCs wear hats or nothing, Creed and Kell, Al'rahem, Lt. Mira (in Space Marine game), and other leaders are without helmets, and I'm not sure if the concept of "helmet" has even made its way to Catachan, yet.
Please note, I'm not trying to belabor the point, much, but there is a precedent for skipping the helmet; Space Marines are more inspiring, and Guardsmen probably see it as not that much more armor than their head naturally has.
So, since I brought it up, DW: Rites of Battle, p.152 goes into stuff for the perks of a Space Marine suffering from Sanshelmetitus. I'm not going to type the whole thing, but one thing is a +5 to Fellowship-based tests. Would this be appropriate to Only War games? Or is this setting not in need? I suppose they lose a bit less, but they also take a lot less to die.