This lens is another pretty bad performing Sony auto-focus lens in infrared. I do like the 35mm focal length for travel so this is a shame, but thankfully the Zeiss FE 55mm is perfect and it does force you to take nicer images, even if it feels a little constricted at times. From what I can tell the Samyang FE 35mm f/2.8 lens does look slightly better than the Zeiss version for infrared (Thank you to Chester for testing this out for me). Plus it's smaller, lighter and cheaper, while providing pretty much the same IW and focus speed, so that one seems like a no-brainer to me.
If you are stuck with this lens for IR then I recommend sticking to f/4 for landscapes, it's pretty sharp there and performs in the '7' rating at the point. Stop it down much more than that however and the hotspot will start to show. It is never really good enough to give flat inky black skies at 830nm and beyond however.