I took the UPX last summer when I was in Toronto and I wasn't very impressed. It seemed to crawl thru the elevated portions near Pearson. The larger section once it hit the mainline was quite quick but I found the ride surprisingly loud and jolting for a train.
I still think the UPX would be an ideal place to implement battery BEMUs. The station lengths are perfect, the electrical connection infrastructure is already in place thru the elevated portions, and electrifying Union would be easy and cheap as it has it's own station. This added to the fact that it was suppose to be electrified nearly a decade ago.
The amount of non-catenary section is relatively small and hence the BEMUs wouldn't need to have as many batteries which lessens the weight which further increases de/acceleration. It would provide a much quieter and smoother ride and save significant operational costs not having to purchase diesel fuel. ML, at least has said, that they want to explore BEMUs and the UPX is an ideal route to test them on. The number of trains themselves is not very large so they could acquire them fairly quickly as every major manufacturer now produces them.
