IME, and keep in mind that my experience is only with harmala (ground seeds/extract), is that it increases duration but "rounds off the sharp edges". By that, I mean that the visuals are not as intense.
With an IM injection sans harmala the trip is much more detailed/sharper than without...obviously I can't compare oral in the same way concerning n,n,-DMT.
I can, however, with mushrooms and they are not NEARLY as good, IME/O, with harmala.
This comparison was done with my own homegrown cubes (fresh and dried) out of the same mono-tub.
Your results may vary, but I've read several reports concerning shrooms with harmalas where they, too, were less than impressed with the combo.
All that said; mimosa/rue is still, IME, the BEST way to experience DMT.