Agreed on the audible improvements, though I'm not sure about measurable evidence, as I couldn't find any supporting data. Nevertheless, I recently swapped in an op amp called "Oracle II dual" on the Fosi V3, based on the recommendation from VirtualHiFi.
https://www.youtube.com/watch?v=kNDX7SCN6Jc
These Oracle op amps were made in China and shipped from there, so the wait time might be a bit longer. However, they arrived earlier than expected. The build quality seems better than most cheap op amps, which often have flaky pins. The No. 1 pin, which needs to be aligned with the notch on the riser supporting the default op amp, is marked by a dot on the upper face. Ensure you have oriented it correctly. Other than that, the installation is pretty straightforward.
Immediately after switching on the "new" Fosi V3, I noticed a more immersive soundstage that extends beyond my speakers, with noticeably better separation. The bass also goes a bit deeper. However, it still doesn't completely eliminate the undesirable sibilance. But, considering these are $28 op amps, I might be asking too much. The well-known Sparkos 3602 might perform better, but it costs more than what I paid for the amp. Overall, my experience with the Fosi V3, paired with the Oracle II and Wiim Pro Plus combo, is very positive. I would recommend it to anyone looking to explore low-end (pricewise) Hi-Fi gear without reservation.
@asctim As for op-amps, if you just swap different op-amps in and out of a circuit, it is not too unlikely that significant changes might occur that can be audible - and measurable.