I informed a brilliant mathematician / gambler buddy and he sent me the following:
To a crude approximation, the player gets an extra 0.5 unit on
40-45% of hands (I'm going to be lazy and not try to calculate the number of
double downs which are still correct under these rules).
That translates to an expectation of .2 to .225 units per hand
(ignoring the off the top disadvantage (tiny by comparison) for a 20%+ advantage.
It's all but impossible that any plausible play strategy could lose
much. Standing on all stiff hands (against any dealer card) doesn't
cost close to that. There must be more to this (betting limits, limited
time promotion, cheating, additional rules...) that we don't know about.