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The polarized modulated piezoreflection spectra of the first singlet system of TCNQ and BDP are reported. Kramers-Kronig transformations are obtained and polariton energy strain shifts are determined. In contrast to systems with negligible to intermediate coupling, the piezomodulation spectrum for the mixed transverse-longitudinal polaritons show strong deviations from the logarithmic frequency derivative of the corresponding reflection spectra. Progressions in the piezomodulation spectra in the energy region of the stop band are related to multiparticle states. Piezomodulation spectroscopy is shown to exhibit greatest deviation from derivative behavior when solid state effects become extensive.