Present trains have just too many halts. A mixed duronto (skipping SSNP) would take about 19 hrs while an ac duronto would take 17-18 hrs. GTL-SBC track has too many PSRs as it is hilly region (MG was thought to be optimal, then). They did not alter the alignment much to improve curves and inclines.
As is presently being planned, Mumbai-HYB and HYB-SBC HSR would take Mumbai-SBC time down up to 8 hrs. Any ways, that is at least 8 yrs away, and only if BJP rule continues.