you get more valve lift or gain a broader span of valve control across the lobe with a roller trunnion than you can physically achieve with a flat tappit..
so saying you can do the same thing with a roller just doesnt compute.
you can have a steeper ramp which speeds up lift.
or you can move the valve lifter earlier or later in the lobe position in relationship to crankshaft degree of rotation..fwiw..
there is an advantage here..you cant utilise with a flat tappit...
granted a .904 diameter lifter affords us mopars more options..but no where near what a roller offers...fwiw.