Well the argument for cancelling the electrification twice now has been "we've got these bi-mode trains, and them up north don't mind diesel fumes, so they're perfect for the job". If the bi-modes didn't exist, there was clear agreement that the MML needed wiring, and I believe we would be seeing it almost complete by now.
The 810's have been specced to match the 222 performance. While they may have slightly better acceleration under electric mode, evidence is suggesting they're slightly worse than the 222's on diesel - bye bye any chance of timetable improvements - even if you could fit a faster journey in on the bottom part of the MML.