You are doing ungenerous estimates one way, and under selling a known quantity the other.You think it'll be quicker to get off a train at Cheltenham, change to an SRL service (10 min transfer), then transfer again at Box Hill? Journey time currently says 22 minutes on SRL (bound to increase), plus 15 mins from Box Hill to Glenferrie
That's 57 mins.
Cheltenham to Glenferrie on the current network takes 35 minutes in the peak period, that includes the transfer. Even if you take away the transfer time on SRL if it's amazingly good, it's still 2 minutes longer just in journey time.
Cheltenham to Glenferrie typically takes around 50+ minutes.