Try gelato.... it dices the same way as prman (and you can get a 1 proc one for free as long as you have a nvidia display card).
Its not actually that Mr cant render good displacement in equal time than prman (in fact coupled with gi or fg mentalray probably renders much faster than RfM)! its just that most people over tessellate stuff quite badly (ill say last time i debugged this they had over tesselated by factor of 1000,000 meaning they rendered 3-4 times longer). And cranking up options for good quality.... is not the way to go! Thing is most beginning 3d artist havee a hard time disabling all those features that eat time but dint impact the final image. the following image actually renders FASTER in MR than prman (and yes its just displacement):
In actuality the picture above is Damn hard to do in prman without doing a humongous over tessellation or modeling it in (becasue the dicing would cause visible render artifacts on the discontinuity peaks on a Prman style renderer). So switching renderer's don't make your life easier, they all have their pros and cons so while you gain something you automatically loose something else.
So in order for us to tell you what to turn on you'd need to describe the scene descriptor very well. (when you begin you tun everything OFF first).
But yeah prman uses less memory. And quite frankly you haven't rendered until you have used prman Pro server. Tough gelato comes pretty close.
PS rendering is by definition slow