Is there some way to squeeze autoboxing in to make this shorter?
Yes, there is. You can simply write:
double sum = vals.stream().mapToDouble(d->d).sum();
This makes the unboxing implicit but, of course, does not add to efficiency.
Since the List is boxed, unboxing is unavoidable. An alternative approach would be:
double sum = vals.stream().reduce(0.0, Double::sum);
It does not do a mapToDouble but still allows reading the code as “… sum”.