Given the design of powerhouse with built-in turbines compartments and so,  I don't think capacity can be increased later on (as increasing capacity means adding more turbines). One thing we all need to be clear here is the nature of RoR model. Rivers in Nepal have low current (flow) during dry season including Karnali. Some studies have shown that 200-300 megawatt of electricity can be generated based on RoR model. I don't know how GRM came up with 900 megawatt later and I don't think they have made their reports public yet. Its not only head but flow of water too that affects the capacity. GMR could have develop this project as peaking run of river (PRoR) too given ideal geography of the region. 
When we talk about Nepal's portion of power when constructed mutually, it depends on the portion of capital Nepal invests. But this is not the case in UKHP. GMR acquired license for DPR & construction through bidding, given it provided most attractive perk to Nepal (12% free electricity and 27% free equity to NEA). If you think this is not a bad deal and we cannot even hope for better deal than this. But this is not the case. We should be more concerned about the possible development of UKHP as a multipurpose project. Independent Hydropower engineer's research and prefisibility studies have shown that this is possible. 
@ Riddle Yes I went to Dcops sanga, 2058 batch

Jay Nepal