I'd say that at least in the interim, increasing parking rates would be a better income generator than highway tolling, not that we shouldn't do both. Highway tolling mostly effects peak usage, while increasing parking rates is better at controlling discretionary car trips and the decision to own a car (or two). Toronto definitely does have some wiggle room when compared with other North American cities (In Particular, Calgary).
See Here.
The thing with road tolling is, is that people expect an increase in the level of service to justify the tolling. You could widen the highway, or improve its safety and throughput, but then you would end up with traffic flooding onto Downtown Streets with nowhere to go. Improving the traffic flow on the highway would also be counterintuitive to expanding transit. I could be misjudging the way people think though.
I think people could survive without the Gardiner, but it'll take massive transit improvement, $2/barrel oil, and a chunk of it to fall onto LSB for them to realize it.