Plasma turbulence measurements play a crucial role in understanding transport, which affects the fusion yield of tokamaks. However, there is a notable scarcity of plasma turbulence diagnostics, particularly temperature fluctuations. While soft x-ray (SXR) imaging holds promise for measuring electron temperature fluctuations, such a fluctuation diagnostic has not yet been developed. In our prior work (Chen et al 2021 Rev. Sci. Instrum.92 055337), we successfully designed an SXR-based electron temperature fluctuations diagnostic and developed a reconstruction algorithm called pseudolocal tomography. In this paper, we further demonstrate the feasibility of this diagnostic using fluctuation data directly derived from gyrokinetic simulations of NSTX-U L-mode discharges. The reconstruction algorithm has been refined to achieve the same accuracy using a simpler diagnostic layout than in previous work (Chen et al 2021 Rev. Sci. Instrum.92 055337). The enhanced algorithm also enables the reconstruction of electron density fluctuations. In light of this, we prefer to refer to this tool as a ‘fluctuation diagnostic’ rather than solely an ‘electron temperature fluctuation diagnostic’. Consistent with our previous work, this study explores the diagnostic’s range of detection, emphasizing its ability to reconstruct the 2D k–f spectra of both electron density and temperature fluctuations in both radial and poloidal directions, and the cross angle phase between them. Subsequently, we investigate how the diagnostic’s performance is influenced by the region of interest (ROI), diagnostic parameters such as the number of lines of sight and viewing angles, signal-to-noise ratio and finite beam size. Through comprehensive analysis, we establish the feasibility of this new SXR diagnostic to measure turbulent fluctuations, including relevant potential diagnostic limitations such as the diminished effectiveness when the ROI is too close to the plasma edge.