It doesn't work quite that way currently. Note length modifiers are applied afterwards and are not shown in the view. The piano roll does let you override the playback start time and duration of any note, and if you make any manual modifications then the length modifiers aren't applied.
In the fullness of time we expect that the view in the piano roll will more closely reflect the actual MIDI data, however the relationship between the notated notes and played notes is incredibly complex. Also, in the next update the note dragging is much improved (you get visual feedback while dragging, not after releasing the mouse).
Architect & Developer - Steinberg London