Thank you again. I went ahead with this and it is literally the perfect fan. My $199 Sonnet eGFX 350W runs a PowerColor Red Dragon RX 5700 XT and it is absolutely silent with this Noctua mod. The Akasa PSU it comes with is totally silent too which is great and the GPU has a 0dB mode. Could not be happier.
Oh, and I have stress tested the hell out of this thing. When the GPU has the VBIOS switch set to 'Silent mode' rather than 'OC mode' the 350W PSU handles it without a problem (as predicted by itsage over at egpu.io). I had an NVIDIA Geforce GTX 1080 FE previously and the 5700 XT actually destroys it.
Have not tried 'OC mode' and probably won't bother.
The 5700 XT will work in the 350w Sonnet? I was looking into get one for mine to replace an RX 580. Their compatibility documents say that it will only support the 5700.