Right so lets say you do that. Person gets beat and it butt sore. He dogs you, now you have tpo deal with an average or lose out because you pissed off a child.
But if he's butt sore, wouldn't what he give you be lower than the average anyway?
Besides. Only a fool would publish the differential. You give the winners in each category, no specifics. No one can complain if they only know the final score, and not how much it was or wasn't altered by their soft scores on comp. You tell people how much it can potentially change, of course, but there is no need to show, say, anything but your final total of battle points.
I feel it's critical it happen AFTER the game. I don't know if the guy was a slow or inexperienced player or a cheater before the game. The only people who would use after-game comp to screw someone are the same sort of people who it is going to intentionally be hamstringing anyway. It's not a realistic threat with the option to drop an anomalous result anyway.
If you do player scored comp before the game it hurts everyone -- for instance, a player in the vein of Mr. Prometheus would probably be low-balled even if the gameplay or list that day didn't merit it, merely on reputation. Similarly, you might see a guy with some Riptides and give him a zero only to find he's a super nice guy and you had a great game and his Riptides were the only good thing in his army.
Rubrics don't work. People only want rubrics so that they can still be a dick to their opponents but dance around the comp with their list and then act like they were a perfectly nice guy all day because the rubric said so.
I much prefer the player-friendly method. I would rather see tough lists but people not being jerks to one another, than average lists and everyone mad because they couldn't take what they wanted. Not to mention, you can make this additive, so no one ever gets a negative, only no added points.