6 mins will mean nothing to most people! What matters is whether they run or not and on time.
You could say that about any journey time improvement suggested, so it’s not a reason not to aspire to cut journey times.
As to whether 6 mins is realistic? As well as the faster departure from KX, every time the speed limit steps up heading North it will be quicker to get to the new limit, and that includes the handful of places where the limit drops down below 125. And the departures from Grantham and Retford will be quicker too. (And Doncaster too if the services stay on electric until Shelby, which RTT suggests is the case.)
Looking at the 2030 there is currently 4 min of allowance as far as Doncaster as well, which could offer further scope, although that would be at the expense of performance reliability.