I will not try to proof if you should soft mount a flight controller on a racing drone. I will only say, that few months ago I was against it, but lately I changed my mind. Stronger motors, stronger magnets, more torque, more speed and out of nowhere, incredible amount of noise can be fed into gyro signal. Sure, this is not required, but motors, ESCs and battery will thank you when you soft mount flight controller. Less, noise, less restrictive filtering required, lower signal delay, better flight performance.
In most places over internet you can find either a rubber standoff or double sided tape solution. Sure, that works, but there is something better. Dedicated, double sided adhesive, vibration dampening pads. There are many sources, and many names. I'm using Sekisui brand. Check ebay, Amazon, HobbyKing. Look for gyro pads, vibration pads, vibration dampening. I do not want to advertise any particular seller, so you are on your own here.
My way involves some 3D Printing to make proper surface over PDB, but with some imagination different solution can be found for sure. For those who want's to 3D print, here is my design. Bottom part can be attached with screws. To attach FC to upper part, you can use hot glue for example. Somehow M3 thread is not so easy to print…
Final effect? Nice looking, but far more important, very efficient!