The class 90s did it in 86 minutes a few times. I expect a 321 could manage the same so 80 minutes for a 745 doesn't sound too challenging.
Around 52 mins Liverpool Street to Ipswich and 32 minutes From Ipswich to Norwich seem to be times to aim for plus recovery and dwells.
I'm not sure 49 mins (Liverpool St to Ipswich) and V.V can be realistically be achieved reliably or timetabled without further infrastructure improvements to increase speeds and improve the pathing of trains through the various junctions.
Certainly a start-to stop run of under 49 minutes IS achievable with a clear run and NO TSR's.
Would a TOC try to timetable that - especially on the Up line - unless the trains following it off the branches can be timetabled much more than 3 mins behind it. Ideally 5 to 10 mins would be ideal. Otherwise, any delay to the Nin90 could result in signallers allowing the GE stopping services out in front to keep them to time - especially as the bulk of revenue comes from GE commuter trains.
One difficulty on up runs is keeping any GE traffic far enough ahead in the Stratford area so that the Norwich service can sail through on greens. The standard 3 min headways are an issue as most services are allowed close to 15 mins start to stop between Shenfield and Stratford where the pass to pass time is nearer 12 mins. That suggests any train stopping at Stratford needs to be at least 4.5 to 5 mins ahead. Personally, i think the Nin90 should all stop at Stratford due to the vast array of connections there and more efficient use of the available paths.
Current Class 321 sectional running times seem slower than what the 321's can achieve, but experience suggests many of these are in worsening mechanical condition and struggling to make up time when there are delays. The now departed Class 360's were fabulous at regaining lost time.
The Sunday running seems to prove that reliability and speed can only be achieved on the GEML with far fewer trains running.
As for Class 321 vs Class 90, the 321's start away from stations a bit quicker, but the 90's reel them in over 60mph - in essence not much to choose acceleration wise, but 321's brakes are superior. And the 321's start and top better in the wet.