I am running VSAN 6.1 hybrid with P3700s and have been before they were officially supported on HCL. I have not run into issues with VSAN that are related to the P3700. I'm using the HHHL form factor and not 2.5" though so not sure if that makes a difference. Although I generally agree with recommendations to firmly stick with VSAN HCL, I haven't run into any problems at all from P3700s on any firmware versions. With no negative impacts, I'm not seeing a reason to downgrade the stock 8DV10171 firmwares that are shipping with these disks. That said I'm not seeing a reason to upgrade to newer firmwares either when they're released since they probably won't be qualified on VSAN HCL yet.

I'm also not aware of any way to downgrade the firmwares from higher versions to the 8DV10131 that's on HCL.