Here is a NY Times story about his idea. While I don't really think this should be a priority for the city, it does have precedent and is interesting to think about. The above image shows the growth of the southern tip of Manhattan, and you can see that a substantial portion of the area is built on landfill. Considering the value of land in this area building more of it makes sense. Or made sense then, anyway. Now that employment in the area is shrinking and residential population is growing I don't think the economics of creating new land can justify new landfill. Consider that firms outbid households for real estate because they will either benefit from increased productivity, access to markets or can pass their costs on to consumers. Households seek to maximize their amenity value and reduce transport costs. In the case of lower Manhattan, financial firms--the richest firms in the world--are not outbidding households for these lands. So the commercial value is declining to the point that residential uses are more profitable. That doesn't suggest we need more land.
In any event, here is a link to a map of the Bridge's Survey of Manhattan from 1807. The grid is laid out (I believe this is the Commission that created the grid introduced in 1811 but am not positive) and the original features of the island are noticeable. You can also pan and zoom on the map, which is pretty cool.