I love the sword. I switch back to it frequently from my purple lirpa/bat'leth.
"...what kind of damage boost does this thing get when it is "charged" by weapons fire?..."
It can absorb weapon energy blast and use the same energy damage and proc against your next target with a melee attack.
I don't think that there is any way of measuring the damage - this would have to come directly from Cryptic. It's a unique weapon, it works differently from ANYTHING else. The numbers wouldn't help you compare it against anything.