Ok, so I took the plunge and the device seems to be fine!
I went all-in and removed the 900mhz antenna completely (a quick bit of soldering did the trick). We (in Australia) won’t be able to use that antenna anyway so it was entirely redundant.
Having put the Rachio back together, I ran through the setup, transferred my settings from my R2 and did a full test. It all seems fine to me.
DISCLAIMER:
I am aware that I have voided my warranty.
Please don’t try this yourself unless you’re willing to take the risk of bricking the device and/or losing any warranty on the device.