Welding 1.5 Al is not more demanding than welding 2 al mm?
You might want to fabricate some alloy tanks and put that theory to the test!
Getting a uniform non porous weld with 2 mm Al. Sheet material is pretty high on the skill chart, 1.5 is much tougher.
I've built in both and I'm going back to the 2mm. It's just easier all around.
And the little bit of extra weight isn't going to matter with my bikes.
Evan Wilcox also uses 2 mm due to ease of welding . There are a couple of other practical reasons as well. One is that after fairing the tank you thin the material considerably in places. Between the sanding the ewheeling , the 1.5 mm sheet may only be 1 mm in spots, especially near welds.
Good luck trying to reweld that should a leak show up in that thin area. Welding al at 1.5 mm is difficult enough, at 1mm it becomes much more difficult again. It can be done, just as razor blades can be welded, but not by most welders!
Glen