If you want evidence why the city has finally launched a long-awaited planning effort for JP, look no further than this eye-popping prediction: Planners expect the Washington Street corridor to see population growth of as much as 2.5 times current levels. That possibility was among the top topics of discussion last Monday as the first neighborhood review of the proposed development guidelines at the Egleston Square Neighborhood Association monthly meeting. Chaired by Alvin Shiggs and Carolyn Royce; the discussion was led by Sue Pranger. Marie Mercurio, senior planner for Jamaica Plain and coordinator of Plan JP/Rox, was joined by her colleague Tim Davis, senior housing advisor for the Boston Redevelopment Authority, to answer questions and clarify concerns about the draft development guidelines. The planning push is a once-in-a-generation effort to shape the neighborhood's future.