Just had a look at the current timetable, and a typical 'fast' service, say the 08:00 from Kings Cross, First stop York, then Darlington, Newcastle, Berwick and Edinburgh takes 4h 22m. Run time to York is 1h 49m, even if all the bits capable of upgrade to 140mph south of York were to be upgraded to 140mph at 3s a mile my guess is savings would be 5 minutes tops, probably less. York to Darlington is mainly 4 track, and straight, so could be a easy win, but again savings are going to be around a minute, may be two. North of Darlington the alignment of the route appears to be the limiting factor, so if you saved a further 2 minutes I would be surprised.
Given current stops and dwell times you may get down to ~4h 10m at 140mph, is that going to generate a massive increase in business. That stopping pattern for an hourly fast service is probably the least stops that would be viable. 6 minutes dwell at York seems a bit much, but again its only tweaking around the edges,
My view is 140mph wouldn't be the step change that HST introduction was well over 40 years ago (and which I clearly remember), and its interesting that the headline London York time has stayed pretty constant since the Selby diversion was fully operational, that points to the current railway being at the limit of what can be acheived. My view is to get below 4hr London Edinburgh on a regular basis, i.e. most of the hourly fast services then you are going need to build new sections of railway, and in the current climate, and with the issues surrounding HS2 that will not happen.
Although I am using London Edinburgh times any improvements could flow though into other services, the main one being London Leeds, but again they will be minimal. If anything London Leeds is the route that would benefit most from a speed up, ideally to below 2hrs, with 2tph is there scope to speed up one of them, fast to Doncaster maybe. faster running Doncaster - Wakefield, I dont know if 125mph would be possible, line is pretty straight but 140mph south of Doncaster isn't going to make much difference on its own either.
So as a regular ECML user do I want an overall saving of 10 minutes given the cost and disruption of delivering it, no I dont think its worth it, either personally or taking a wider 'GBR' view.
The eastern leg HS2 approach was the right one, but has failed, and I dont see it, or anything similar being back on the agenda for many years.
The ECML issue that is going to constrain services is the capacity for growth, not journey time improvements.