Acrylic cutout letters should be mounted with studs and glue blocks. That has been the industry "best practice" for a long time. This job has the added problem—really the main problem—
of dissimilar materials—acrylic and metal cladding. Dissimilar material is often a cause of adhesive failure, regardless of weight. A mechanical fastener is the only really reliable installation method outdoors.
You may get away with adhesive only—modern adhesives are better than in the past—but how much time will you really save? If one letter falls, a single return trip negates any savings. And if one letter falls, there are sure to be more.
On the other hand, I'm all for experimentation. That's how discoveries are made. But I prefer that others do the experimenting.
Brad in Kansas City