If drum brakes are in good shape they will stop your car just fine from 80MPH down just as well as discs will for 1 or 2 times. If you hit them above 80MPH or so (especially more than once) the discs will outperform them.
My last two factory Hemi cars have had the standard 11 inch drums and both cars have stopped great, my 66 is non-power and it does not need to be upgraded. I do not drive on an interstate in heavy traffic above 70 MPH though. It really depends on vehicle usage.

Sheldon