"Efficiency" is ambiguous. In terms of (benefit dollars delivered)/(benefit dollars delivered + administrative costs) its almost true-by-definition, since the absence of means testing removes most of the source of administrative requirements in traditional welfare programs.
In terms of effectiveness at achieving the goals of welfare programs, it is far less clear, though there are pretty clear arguments that certain features of basic income -- particularly the lack of disincentives to outside income -- are beneficial in that regard. OTOH, there are also pretty clear arguments that the lack of need-based focus -- which is intimately tied to the lack of disincentives -- are potentially negative, especially when replacing welfare programs whose existing qualifications are based around special needs (e.g., programs qualified by particular disabilities) that increase costs rather than simple lack of resources (e.g., income/asset-qualified poverty support programs.)
