Adding additional cars to trains I agree would probably be better for optics than converting an existing car. As for the perceived 'inequality', I would point out that Canadians readily accept first class services as a fact of life on other means of transportation, including government-owned ones (VIA, and quasi-government Air Canada).
To offset some of the potential unhappiness with the new service, maybe institute a fare freeze or a minor fare reduction. Or perhaps announce lower off-peak fares to encourage more ridership on those trains.
And yes, seeing how UPX performs would make sense, although that wouldn't be directly comparable, because UPX doesn't have the "coach class" component to it. Maybe the solution is mixing in a couple UPX-like trains on the Lakeshore lines during AM and PM peak, so that GO+ customers have a 'premium' option? Make it an 'on-demand' stop service, so it potentially speeds things up?