IDK why I said PATH so you can laugh at me there. And again, is it really worth adding another set of LRV types (we already have 4 either here or coming (Flexity Freedom, Flexity Outlook, CLRV (Being retired), ALRV, Citadis), possibly 5 depending on the streetcar expansion).
Given the high ridership demands on the 510, I believe it's worth consideration. The 510 is the fifth busiest route on the TTC network, moving 55,000 people daily; it's busier than any bus route, and has more daily riders than the Sheppard Line, SRT and the Scarborough Subway.
Ridership on the TTC
streetcar network is expected to skyrocket in the coming years, and this ridership increase will be exceptionally pronounced on the 510 Line, as it runs through King and Spadina, which is Canada's fastest growing neighbourhood. The population of that neighbourhood will increase 2.5x by 2031. The LFLVs can theoretically boost the capacity of the 510 by 1.75x, to around 96,000 daily riders, and I expect that most, if not all, of that new capacity will be saturated by high passenger demand.
To put things in perspective, the Eglinton Crosstown Line is expected to have 180,000 daily riders to 2031. Given the "natural" ridership increases the 510 will experience due to densification at King/Spadina, and given the number of riders that would be diverted from the University Line, it is completely conceivable that ridership on a Spadina subway line would exceed the daily ridership of the Eglinton Crosstown (however, I would expect peak hour demands to be lower on Spadina, since its ridership is more evenly distributed over 24 hours).
Furthermore, with the potential for long term capacity constrains on the University Line (I'm talking 2030s to 2040s timeframe), something will need to be done to relieve demand on that line, and a subway on Spadina is the most obvious solution.
If the price to achieve this includes buying LRVs specifically for Spadina, it's worth it. The SRT has had unique rolling stock for 40 years, and its ridership is a small fraction of a potential Spadina subway.
Finally, I'll emphasize that I'm not saying this is something that should be considered today. This would be a project for the distant future, probably not until the 2030s decade.