In case the HDD demands more current will my Pi be able to draw that extra juice from the PSU and supply it to the HDD through USB 3.0 ?
Your RPi will never draw that much current, and so the over-capacity of an 8A-rated PS won't, in and of itself, provide any advantage. Here's a list of the "official" power requirements., and this note seems to address your question specifically:
"If you need to connect a USB device that will take the power requirements above the values specified in the table above, then you must connect it using an externally-powered USB hub."
Which is to say, if your USB requirements exceed the limit in the table, you'll need to have a powered hub.
if yes, is there some hard limit of how many Amps the Pi will supply over USB 3.0 ? and is there some risk/hazard in doing this ?
The limits to current flow into the RPi are based somewhat loosely on USB specifications, but more definitively by the RPi's minimum voltage requirement (4.63V), and the voltage drop imposed by the wiring used to deliver the power to the RPi's input.
Re the risk/hazard: The only potential downside to using the supply you've chosen is in its ability to deliver the required voltage across the range of loads your RPi will draw. Power supplies produced by responsible manufacturers will include a set of specifications that will give you this information; it may be called load regulation, and it simply reflects how much the voltage may vary across a range of loads. It's not unusual to see power supplies that do not regulate their output voltage very well at very low output currents.
