We studied how a surface-micromachined Fabry-Perot interferometer, realized with Si / air-gap distributed Bragg reflectors, would perform at the middle-infrared wavelengths. Compared with traditional solid-film pairs, this Si-FPI technology features better index contrast, which enables wider stop band and potentially higher resolution. Four different designs of interferometers were prepared and compared. Two designs apply the solid-film reflectors of Si/SiO2 structure. Their data is exploited as a reference of a middle-infrared interferometer and, as a template for mapping the performance from the simulation results to the measured data. The third design operates at the thermal infrared and it was our first embodiment with the Si/air-gap mirrors. The performance, reported earlier, is here referred to for estimating the technology scalability down to shorter wavelengths. Finally, we realized a non-tunable proof-of-concept version of the Si/air-gap technology for middle infrared. The measured data is mapped into an estimate of the achievable performance of a tunable version. We present the transmission and resolution data and argument the simulation models that reproduce the data. The prediction for the tunable middle-infrared Si-FPI is then presented. The results indicate that such a device is expected to have two-fold better resolution and a clearly wider stop band, compared with the prior art.