A while back I discovered I had misinterpreted the Rogue Planet rules in regard to Power Weapons. Well after some deep thought and further playtesting, I think we're going to stick with the same as a House Rule.
The official Rogue Planet rules regarding Power Weapons is that they provide a +1 bonus on all melee (CQ) skill checks. I had (erroneously) read that a Power Weapon boosts its wielder's CQ by one e.g., a Chainsword equipped Imperial Guard Trooper's CQ would be considered 4 instead of the usual 3 for combat resolution purposes.
I've decided to keep the CQ boost interpretation. Why? It is actually to do with how damage is resolved in Rogue Planet.
Depending upon the (1) Skill Check result and (2) the Target's Armour, a single attack can cause multiple points of damage. For example, a Melee attack that results in a Partial Success or Total Success against an opponent whose Armour rating is Light results in "...damage equal to the absolute difference between the relevant skills being used by the combatants [CQ, RAT, DEF]. If the combat skills are the same, then a single point of damage is done."
Power Weapons already have the Armour Piercing trait (lowers Armour ratings by a factor of one e.g., Medium is treated as Light for damage resolution). With a boost to the combatant's CQ provided by the Power Weapon, coupled with the reduction of Armour value, Power Weapons become materially more effective, especially where the CQ boost lifts the wielder's Skill 2 or more points above that of the target. That can translate to inflicting 2 or more points of damage in a single attack, which can be hugely important when engaging with high-value targets and groups. It is a significant effect and I feel it fits perfectly with the R40k setting.
So our house-rule reads like this:
For the purposes of Melee resolution, a unit attacking with a Powered Weapon is granted a +1 CQ boost. This is only applied when initiating a CQ Attack and is not available nor factored into the Unit's defence when attacked in Melee.
For example, an Imperial Guard with a Chainsword is attacking an Ork in Melee. The Guard's CQ rating of 3 is boosted by +1, lifting it to 4 for the purposes of executing a CQ attack.
This bonus is in addition to any other modifiers detailed in the Rogue Planet rules e.g., Chainswords are provided with a +2 vs Medium armoured Units.
When the Ork attacks back, the Chainsword does not boost neither the Guard's CQ nor DEF rating.