WP can be very effective as an obscurant to vehicle crews, and WP M64 probably among the best choices in that it stays in place and continues to burn as long at it has fuel. a Closed down tank already is blind to within 25M usually and now you made it that much worse. Add to that fumes and any particulates that work past the Mantlet and yeah you can see why a crew would want to unass quickly. the M3 75mm Gun was quite effective on the Bulk of the Panzerwaffe which was PzKpFw IV and Stug III/IV. Where it fell was in pentrating the Panther and Tiger Series.