Upgrading mechanical disk brakes
As the comment mentioned, if there is an issue it will be with the fork.  The rotors are designed to withstand the additional braking loads.  However, for what it's worth, I've always felt that larger rotors were easier to bend in a crash.  There's more area to strike on a rock or something, and you get better leverage on the rotor.  But there are plenty of people who think that's just superstition.

I think you might find that you don't need the larger rotors, though.  I switched from BB5s to BB7s (and then to SLX hydros), and felt that the BB7s stopped harder than the BB5s, even at the same rotor size.  And I was using crappy levers, too.