the optical cavity mode does not perfectly match that of the external modes used for measurement and signal injection (see supplement). We thus expect $\eta_\mathrm{M} = \epsilon (\kappa_\mathrm{ex,o}/\kappa_\mathrm{o})( \kappa_\mathrm{ex,e}/\kappa_\mathrm{e})$, where $\kappa_\mathrm{o}$ and $\kappa_\mathrm{ex,o}$ ($\kappa_\mathrm{e}$ and $\kappa_\mathrm{ex,e}$) are the optical (microwave) cavity linewidth and external coupling respectively, and $\epsilon$ parameterizes the optical cavity mode matching. The fit result, $\eta_\mathrm{M} = 43 \pm 1\%$, agrees with the theoretical expectation, $\eta_\mathrm{M} = 43 \pm 4\%$ obtained from independent measurements of $\epsilon$, $\kappa_\mathrm{ex,o}/\kappa_\mathrm{o}$, and $\kappa_\mathrm{ex,e}/\kappa_\mathrm{e}$.

Tuning the membrane position *in situ* within the optical cavity changes both $\kappa_\mathrm{ex,o}$ and $\kappa_\mathrm{o}$ due to interference effects (see supplement), and therefore alters the matched conversion efficiency (Fig. \[sparam\]c). Matched conversion efficiency initially increases with $\kappa_\mathrm{o}$, reaching a maximum at $\kappa_\mathrm{o}=2 \pi \times 2.7~\mathrm{MHz}$, and then decreases as internal optical cavity loss begins to dominate. The *in situ* tuning of the optical cavity also changes the optomechanical coupling and thus the achievable converter bandwidth $\Gamma_\mathrm{T}$. For intermediate linewidths near the highest efficiencies, the optical cavity became unstable, possibly due to large optomechanical coupling in these regions. The peak conversion efficiency achieved was $47\pm1\%$, approaching the quantum feedforward threshold efficiency. At peak conversion efficiency, the matched converter bandwidth was 