"He who marches out of step hears another drum." ~ Ken Kesey
I think your print will look better on top of an underbase, you will use less ink if printing on top of an underbase and you will not have to stop to clean lint out of your screen if using an underbase. White ink is $35 per gallon, what does shimmer cost? I vote for using an underbase, you will be done printing in a little over and hour (I assume you are printing auto with those quantities but maybe not).