RH-GO might be a bit faster, but the trade-off for the Ontario Line here would be more destinations, more connections to other routes, and higher frequencies.
I have made my opinion on RH-GO known in the past on here. If we went with that scenario and sent the Ontario Line to Langstaff, I would simply discontinue RH-GO service. It's expensive to operate, very low ridership, impractical to upgrade, and doesn't allow for connections to East Harbour, Bloor, Eglinton, and only an impractical one at Sheppard. So in this fantasy scenario, we would only end up with two services (Yonge Line and OL) that serve different areas.
Space on the Bala Sub is the biggest issue. Looking at the corridor on google maps it seems wide enough, but without a study, can't say.