I'd be very concerned getting 15mile nNLOS clients working at all. This is going to be a tough project for you. I'd suggest using a KP KPPA-900DP-FP 13.5dBi flat panel and use it as a small high-gain sector antenna, and then use their KPPA-900DPY17 17dBi yagi's on the client side. I hope your 900 spectrum is really clean, if it's not... then I wouldn't even bother with this project... it's going to be a big waste of time and money.
ALSO... LinkPlanner is good for planning LOS shots... but it's not going to give you reliable nNLOS results in most cases, even with clutter data... because the clutter data doesn't take into account the many different types and density of clutter... e.g. are they pine trees? leafy trees? is it winter or summer? etc...