I have a BlendMount for my ScanGaugeII with the gauge mounted on the top side of the rear view mirror and my V1 is mounted by suction cup to the upper edge of the windshield to the side of the rear view mirror. And unless your rear view mirror is REALLY big, the V1 will not look that small under the mirror. The V1 is definitely not small by any means... On the other hand, that mounting location would allow good angles for the radar detector to pick up signals.
So, I can't speak specifically about the V1 BlendMount, but their ScanGauge BlendMount is high quality stuff and easy to install. Personally, in my car, I would not install the V1 under the rear view mirror just because my rear view mirror itself already blocks some vision and I can't image mroe being blocked and having to look around it. But that is in a Crown Vic, not a T4R.