We performed a small-scale demonstration at GSFC of high-resolution X-ray TES microcalorimeters read out using a microwave SQUID multiplexer. This work is part of our effort to develop detector and readout technologies for future space based X-ray instruments such as the microcalorimeter spectrometer envisaged for Lynx, a large mission concept under development for the Astro 2020 Decadal Survey. Using a ROACH2 FPGA at room temperature, we simultaneously read out 32 pixels of a GSFC detector array via a NIST multiplexer chip with Nb coplanar waveguide resonators inductively coupled to rf SQUIDs. The resonators are spaced 6 MHz apart (at ~5.9 GHz) and have bandwidths of 300 kHz. Using flux-ramp modulation frequencies of 160 kHz we have achieved spectral resolutions of ~< 3 eV FWHM on each pixel at 6 keV. We will present the measured system-level noise and maximum slew rates, and briefly describe the implications for future detector and readout design.