hi danny,
yes, you would need to figure out the memory consumed by ITS out of the global memory.
Amount of global memory allocated is defined through profile parameter "em/global_area_MB"
ITS memory consumption is quiet transaparent and you can see it in tcode SITSPMON.
Refer to SAP Note 1888428
(Some more knowledge : How to estimate the memory consumption of the internal ITS - Wiki - SCN Wiki)
Personas is not supposed to do load balancing itself. personas consumes the services like restgui defined in your system, and surely you can use web dispatcher, but you have to configure that.
Sushant