Hi, I use the Motorola Link Planner extensively at my job and I can assure you that its estimations are more than precise if you refer to Motorola products. Motorola field installations keep on confirming this.
In the last version of Link Planner Motorola has included the new serie PTP250 that is basically a 2x2 Mimo system that feeds a dual pol 23dBi patch panel. Not far from Mikrotik last systems. When I need to figure out expected performances for MT 802.11n systems I usually start simulating the same link with PTP250 adding later some corrections to keep in play the fact that R52NH are not "exactly the same" as motorola radio modules...
To get a real or, at least, expected real estimations, I usually set interferences to values ranging from -85dBm to -82dBm and I consider an extra loss on MT systems of about 5 to 10dB: for example if Motorola estimates 50Mbps using integrated 23dBi I assume to get the same using 28dBi dish. (28-23 = 5 x 2 = 10)! These kind of estimations usually leads to estimations that are worst but near than what I really get in the field.
Which is the maximum EIRP power you will run the link? Which type of antennas are you going to use?
If you provide me with these infos I can try an estimation for you.
Have a nice weekend.
f.