I don't understand the obsession here with building the line to TTC Loading Gauge. We're talking about a completely new line (not an extension), that will not interline at all with existing lines. There is no reason that the design of existing TTC lines should constrain the OL design.
Overhead Electrification is proven for metro applications. It is the preferred method for new constructions because it allows for higher voltages than Third Rail, and does not create an electric shock risk at track level. Nearly every metro line in Spain, India, China and South Korea uses overhead electrification. Many of the world's largest metro systems have 1.5kV electrification, including Madrid, Barcelona, Milan, Istanbul, Singapore, Tokyo, Osaka, Beijing, Hong Kong, Shanghai, Seoul and Guangzhou. 1.5kV is also the voltage of most Japanese suburban rail lines, many of which have subway-level frequencies. It is clearly fit for application to the Ontario Line, and has many advantages versus 600V Third Rail.
Using a narrower loading gauge and shorter car lengths reduces overhang on curves. This is valuable when designing elevated structures, which the line will need to Cross the Don Valley and the Don River West Branch.