I have had the ScanStrut backstay mount on my Cal39 for 4 years and it is doing exactly what it is supposed to do. I have the ScanStrut forward of the backstay and radome also forward of the backstay. Just be sure you have plenty of clearance so you don't have to worry about the boom hitting the radome in an accidental jibe.
I installed my radar on the backstay for the same reasons you point out. Plus the additional benefit of it being a much easier installation
Just a word on ScanStrut. You can look at the other backstay mounts but you can't find one that is engineered or built as well as the ScanStrut. It is a really nice piece of equipment
If you decide to go with the ScanStrut drop me a note and I will send you the revised installation
instructions I developed. The instructions they provide with the unit makes the install more difficult than it needs to be. The job can be accomplished in a few hours by one person with the minor changes I made. I have cleared the installation procedure with ScanStrut.