An LRT Crosstown in the north of the city cannot be effective, since the city is very wide in the north and a trip on LRT will take too much time. The estimated trip time on the proposed Finch / Sheppard LRT (that incorporates the existing Sheppard subway tunnel) from Meadowvale to Humber College will be 1 h 40 min one way.
We do not have money for a fast crosstown route in the north. Then, it is better to focus on the realistic travel patterns, and optimize connections from the east and west to the center of North York around Yonge, and the two N-S subway lines (Yonge and Spadina).
IMO, the best configuration is:
- Sheppard subway from Downsview to Agincourt GO
- Sheppard LRT from Agincourt GO to the Zoo
- Finch LRT from Humber College to Yonge, with possible further extensions to the airport and to the east of Yonge
If there is no funds for the eastward extension of Sheppard subway, then just start Sheppard LRT at Don Mills.
Between Yonge and Downsview, the saving from using LRT is trivial, and will not offset the cost of converting the existing subway. If there are money for the conversion of the subway, it is better to use them to extend the subway to Downsview. Of course, this should not be treated as the highest priority project; there are more pressing needs elsewhere in the system.