A little push-back here: this precludes batched CPFP, which is somewhere in-between single-tx exo fees and single CPFP exo fees, depending on how many “well I was just making txs already” cases are in play.
I suspect if we really wanted to squeeze every byte(I’m unconvinced, but this thread is for people who are interested in doing so 
), you’d want to support both this type of sponsor and the more general original one, with the associated complexities involved.
(or we figure out a softfork for robust single tx exo fees, with stacking ala @rustyrussell dream world!)