If the crossings were sorted, the signalling upgraded, the OLE retensioned, the power supply strengthened, the track maintenance enhanced (considerably), some bridges rebuilt, some embankments strengthened, places of (staff) safety provided for 100mph+ operation, some track renewed to 100mph+ standards, some of the curves realigned, and some junctions removed altogether, then most of the route is fit for 125. Exceptions are through Chelmsford, Colchester, Manningtree and Ipswich station areas but trains would be stopping there.
Assuming 125 where it is currently 100, and a train of Pendolinesque characteristics, the journey time saving London-Norwich is approx 8mins on today's stopping pattern. Roughly half this saving is due to the better acceleration / braking of the train, and half is due to the raised line speeds. This assumes that all the slower trains in front are shifted out of the way, these being the ones that call at wayside halts such as Shenfield, Witham etc.
With new trains it would be an awful lot of money to save 4 mins. Alternatively knock out a stop for free.