I've used the Prime X Strung during a downhill half (2k feet drop) and didn't have any issues. For reference novablast 1 is one of my favorite shoes of all time though
I've done two runs in the VF3 so far. 5k last saturday and a track workout yesterday. The ride definitely lost some bite compared to the VF2 so wouldn't be reaching for these on short distance road races. Could definitely see the added cushioning help during the last 10k of 26.2 though...
Cadence is referring to steps per minute. Optimally u would be at 170-180ish. Don't think u should be running in the vaporfly 3 until u get a little more experience running in non plated shoes. Have to build up to them.