In the same study that mentioned only 23% of transit trips starting in Scarborough are heading downtown, it was also stated:
"It is likely that there would be significant additional transit use within Scarborough if the local network was improved given the current low usage of transit for internal trips."
That would mean the 23% figure would actually drop if people in Scarborough had access to a comprehensive network that included rapid transit.
The SSE would be a wonderful project assuming there were no other options for getting downtown and we had unlimited funds to spend on transit. Unfortunately, neither of those is true.
Those heading downtown would see a much greater benefit from improved GO service.
The $6 billion for the SSE almost guarantees there won't be anymore significant transit dollars invested in Scarborough for a long, long time. Expecting $6 billion on the SSE, another $4 billion (it was $2.5 billion in 2011 so I'd say this is the minimum it would cost now) on a Sheppard extension and another $3 billion for an Eglinton East LRT is completely unrealistic. $13 billion+
just for Scarborough isn't going to happen.
@robmausser suggested a very sensible plan. How about this:
1) Refurbish the SRT
2) Make improved GO service a priority
3) Extend the Sheppard Line with a stop at STC, ending at or near the Zoo/Rogue National Park
4) Take the $6 billion for the SSE and build an Eglinton LRT that intersects with the Sheppard Line and goes up to Malvern, along with an LRT from Kennedy heading north (potentially with an STC connection). Or, take the $6 billlion and add it to the OL/RL budget, and build it underground to Don Mills.
This would all provide dramatcally improved service in Scarborough, and much greater access to downtown than the SSE alone.
Even if they only extended Sheppard to STC and the Eglinton LRT to Malvern, it's probably not much more expensive than the SSE, and provides far greater value.