Bigger gap = hotter spark = better burn.
GM/Holden set the plug gap at 1.5mm, clearly they did that for a reason. If you don't like that then you best contact Holden/GM and tell them they got it wrong.
Yes but too much of a gap is actually a bad thing. 1.5mm is obviously fine. But 2mm would be excessive. Why do you think drag cars go for amps and duration to increase heat? Because a long spark will make a long initial flame front which can collide against it's self and cause knock. The Mercedes and Chrysler dual spark time their events via spacing so don't suffer from this.
But my whole point has been that the longer the gap the hotter the spark yes but the hotter the spark the more wear on the plug. This is why most standard plugs are .9mm to 1mm and iridiums are are 1.1mm in like 90% of cars. Seeing as a buick V6 has an ancient 60's design head I can't understand why it would benefit going from 1mm to 1.5mm.
You say you get better economy so maybe that is true I have not tested this so I can't comment on that.
But it seems the plugs losh has are nearly out of stock so you have the option of iridium or standard tungsten.
I was just suggesting changing the gap to try and improve wear on the plugs.