200 K/h will be cheaper and improve reliability for freight. It won't attract meb-syd direct traffic like high speed will and isn't that the market you want.
The key to answering that question is simple, its not just the train that will improve the timings, but it will be the track alignment along with the trains speed.
When the XPT was under trial, a police radar was set up on the southern line where it was recorded at doing a bit over 180Km/h on that old track south of Wagga Wagga.
What is the average speed as it stands now for the XPT between Sydney and Albury? The distance is approx. 640km's with the TT taking around 7 1/2 hours, once the track is realigned and if it isn't then it becomes another missed opportunity for freight and for regional passenger rail.
By having an almost straight alignment south of Campbelltown along with gradient reductions, rather than the current long hours the TT could easily be reduced to around half the time it is now, or around 4hrs overall. What takes place south of Albury will be determined by the Victorian government but you are looking at another 3.20 hrs so far on the current TT, if it can be at least cut in half and the capitals to capitals are higher speeds making it more competitive with Flying depending on the ticket prices.
Some time back an survey was conducted on the basis of rail vses flying to Melbourne. At that time the XPT ran via Strathfield so the figures are a bit different now depending on where one lives. Flying to Melbourne meant the passenger had to be transported to the airport, time factors ranged depending on where the passenger was travelling from and by what means. Often driving to the airport was longer than going by Train, especially outside of the daylight/business hours, usually a need also to be at airport an hour before departure was needed, departure being the time the plane was taken off its moorings. Then around 15minutes on average until take off. On arrival at the Melbourne airport it could be up to an hour getting through the barriers and then catch taxi or bus to the city, at least one hour. On average the flight was a bit longer than the scheduled 1:25 for most flights.
Thus for the passenger flying to Melbourne and likely the same in opposite direction was around the 4 1/2 - 5.00 hours mark. Much depended on traffic conditions if driving and many still use the train to get to the airport as the services are usually pretty good between the two airport and Central. Bare in mind though that these times did not take into account the walk and distance from the train, or from car park/drop off point. Other things that extended the times for flying included the wait for luggage to come down to carousel, then to taxi, bus or transport to destination.
Rail at a general average speed of 160Km/h with ability to run up to 200Km/h will be very competitive. Although as I said above the price factor will need to be considered, and here is where rail could offer discounts for families travelling and other incentives to compete. Ticket purchase includes the fare on each end of the trip by bus or local rail metropolitan/city services.