Because the qualities don't stack up, its still utterly worthless.
Your Lasgun is Reliable. Good Quality makes it Reliable, removing Reliable, and is thus worth absolutely nothing. Overload immediately causes Unreliable, whether the weapon was Reliable, normal or Unreliable to begin with. An Integrated Good Quality Lasgun on an Enhanced Potentia Coil firing in Overload would still be unreliable, and this drives me up the bloody wall.
Just use a Triplex in Incinerator-Mode instead.
Or, if you'd rather houserule a bit less 'suck', my group uses this scale of reliability: Reliable>Normal>Unreliable>Scavenged[as per the Black Crusade downgrade]. Good Quality improves your ranged weapon one step, Best by Two [we don't use the "never jams or overheats" rule. Sounds harsh, but I'm the plasma-user and I suggested it]. Acquiring a poor quality Unreliable weapon by any means other than "off the still-warm remnants of a target when in hostile territory with critical levels of remaining ammunition" is just your own damn fault, then.
If already Reliable, you gain +1 damage instead to the gun. Best on an already-reliable weapon does in fact do nothing still, by virtue of a weapon so simple or well-crafted to begin with attaining effective perfection that much more easily. When you get "good" quality lasguns, you're really getting "BEST" quality lasguns; its just a common enough tech that there's just no excuse for poor ones.
So the "unreliable" overload setting just returns it to normal reliability, keeping the bonus damage, for that set of shots.