Catalysis of the reaction: 3-methyl-2-oxobutanoate + lipoamide = S-(2-methylpropanoyl)dihydrolipoamide + CO2.
3-methyl-2-oxobutanoate:dihydrolipoyllysine-residue (2-methylpropanoyl)transferase-lipoyllysine 2-oxidoreductase (decarboxylating, acceptor-2-methylpropanoylating)