I bet you lose 3 db with that thing in place. Which, IMO, is NBD so long as you tune with it in place.
(I lose 4 dB with my floor in place, and for me it's NBD since I can blow over wind noise easily enough anyways. Gain down and move on, for my car.)
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I'd still rabbet out the floor before upholstery, if I were you. Just cause.