The obvious sections for 160kph running without having to spend the earth would be Avenel to Longwood, Euroa to the base of the Glenrowan hills and Wangarrata to Springhurst. This would require newer high powered rolling stock such as a long distance version of the Vlocity even then for a stopping all station service the time saving is not that much.
I just looked at Vic Sig and calculated the three areas you mentioned were 110kms in total. Add the slowdown for the stations along the route and I see your point. The question is having asked for 130kph speed, will V Line actually run loco hauled trains at that speed? Or will they top out at 115 -120 kph?