To be fair, the $27 billion includes 1,025 new subway cars (which tend to cost more than $1 million each) plus extensive renovations to 31 stations. Also, the "open gangway" design in combination with the other improvements (wider doors, collapsible seats) will reduce crowding on trains, which in turn reduces crowding on platforms and dwell time. During rush hour that will have a noticeable impact on travel times. Outside of CBTC, which would allow running more trains per hour, there aren't really other ways to make the existing system run faster.
I do think the $27 billion tag is too high, though.
I do think the $27 billion tag is too high, though.