Modelling extreme rainfall events: 'blanket approches' when considering soil infiltration
I would be surprised if the rainfall-runoff relationship would show a linear relationship for extreme events. I would consider that to be a blanket approach, if you mean using a constant coefficient. So consider using a higher order relationship.

Another thing to consider is whether the rainfall runoff relationship depends on the soil moisture. Maybe check out how other models like SWAT deal with this.

To be honest, I think that the amount of soil infiltration will be insignificant for extreme rain events because it occurs too slowly compared to the rainfall rate and other uncertainties.