Of course it's not always a safe assumption.

But in this case where the design target (device and therefore power budget) was known years in advance, it's not a completely new architecture, they've done a CPU on the same process before so none of the great unknowns apply.

I'm sure they would have realised soon enough if they overshot their target massively and then went for higher frequencies and lower voltages to use that headroom and stay within the power budget.

Really there's only two things we'd need to know:
1. tau vs voltage for 16FF+
2. stock voltage for the A10.

Of course that still won't quite be the maximum but we'd have a minimum figure as to how far you could push it with a desktop/server power budget (essentially unlimited in this case).