almost instantaneously pressurize as soon as the TB butterflies shut. With equal pressure throughout the entire pipe the placement of the BOV is irrelevent. Momentum wouldn't seem to be an issue since the pressurized air is alwasy flowing into the TB under boost. But you're probably correct about the insignificance amount of the air's mass.
Jonathan Howlett AIM-JRHZ32 1993 Ultra Red Twin Turbo 1990 Pearl Yellow Twin Turbo(Deceased)