Is there a generic algorithm to optimally combine elements by some arbitrary scoring method?
In your formulation, you need to compute score of each reduced list individually. So, if there are 2^(n-1) possible reduced lists, you need to perform at least O(2^n) operations.

Unless you can figure out some restrictions or relations between scores of related lists (such as "longer lists always have better scores"), it will remain O(2^n) problem.