18/6 here.
The additional 6 hrs of light is 360 hrs over a 3 month window... multiply by 29 cents a kilowatt hr... or an added 104$ a run... the minor gain in weight is well offset by the cost. That raises the per gram cost by .20 cents... or about 1.15 to 1.25 a gram of finished product. I need things to run between .70 / .90 cents a gram to stay running.
Here in California, power alone runs me just over 300$ on a 4x4 18/6 per run. Add in the nutes, soil, genetics... filters for the RO system etc, currently a run costs just over 600$. At some point, you start watching your $.
Running 24 on isn't going to reduce your run time any significant amount .. been here done that all before.