Hi,
I also have the wifi issue with RPI3 and HiFiBerry DAC+Pro.
Wifi on RPI3 works fine. At least as long as the HiFiBerry DAC+Pro is not powered up. It is very stable, and latency is fine. The problem only appears when the DAC is powered up. I'm not equiped to test the power fluctuations on the RPI, I don't know if it's a problem of power, or a bug/conflict between HIFiberry DAC+Pro and integreated WIFI.
My power supply is "official" 2.5A PSU. I tried with a 2.1A PSU as well, and with a 2.1A output powerbank, all with the same result.