*EDIT* - Just wanted to throw this out there to clear up any confusion:
I tested the 715 and 510 side by side on a busy street and the radar performance in traffic was IDENTICLE. Vehicles appeared at exactly the same time.
It sounds like my 510 was just overly sensitive compared to others, as it would pick me up walking slowly through a room. Based on this, I'm keeping the RCT715, as the radar seems to be working the same where it matters.
After some testing I've decided to stick with the RTL510 and return the 715 since I haven't had it on the bike yet. My 510 has (never) had a false negative. The 715 seems to do so consistently when testing as a runner coming up on the device.
My old RTL510 is sensitive enough to pick up a runner if I'm standing still. I tested the 510 and 715 next to each other. If I do a brisk walk or jog, the 510 picks up my body from pretty far back. The 715 won't pick me up until I'm about 5 ft (right up) to the radar, if I run. And then it would stop detecting me for several minutes, then it would randomly pick me up again 1 out of 5 times at full sprint (once I'm already running into the radar).
Anyway, I found a review on amazon stating that it generally worked great but every now and then would not detect a car until it was within 30', as opposed to 150'. Based on my "run tests" I get the feeling I'll run into this as well. Granted, I wasn't testing with cars, but I actually want the device to tell me if a runner or biker is coming up behind me. I'm not going to risk installing it for a ride since the store only takes new condition returns. Perhaps the 715 is only meant to pick up cars? Or a Firmware update might increase the sensitivity.
I will say this: after updating the firmware as of June 7 2022 - I have zero problems connecting my Google Pixel 6 Android phone to the device and viewing video clips. It seemed fairly quick for me.