master cylinders on bikes with dual discs are 11/16 bore, master cylinders on bikes with single disc are 9/16 bore, I know it is fiscally possible but what I want to know what is the harm, if any to doing this?

 

The only possible downside might be a loss of modulation due to more fluid getting to the caliper quicker. I can't think of any other issue though.

 

I heard the brakes can lock up quicker.....

 

Yeah, but that may not be a bad thing. I'm pretty sure PM recommends an 11/16" bore if you use their differential bore calipers (even on single-disc applications). Assuming you've still got some lever travel, it's just a matter of learning the feel.