Research on flu a- and pre-symptomatic transmission seems to say that it is not a large part of transmission (10% of cases are asymptomatic, and they shed less and less days). See references inside ref (1). Of course, if we were to decrease symptomatic cases by SD, then remaining flu threat will be largely asymptomatic…
Time from infection to symptoms is 1 to 4 days for flu and 2 to 14 days for COVID, so pre-symptomatic periods differs also.
R0 before public health measures for flue is between 1 and 2, for COVID 2 to 3 or even more
(1) Patrozou, E., & Mermel, L. A. (2009). Does influenza transmission occur from asymptomatic infection or prior to symptom onset?. Public health reports (Washington, D.C. : 1974), 124(2), 193–196.
link removed (surprise)