Couple of issues to flag with 80X's if Abellio go for them, though the second would be applicable to any unit introduced, bi-mode or electric:
- Well-documented acceleration issues away from the wires - they'd have to match a Meridian on the new more intensive timetable, which has superior performance to a HST (something like 0.75m/s/s compared to 0.55m/s/s IIRC - GWR's variants don't even match the latter), or passengers would have to see increased journey times to cope. Bombardier have claimed their variant will acheive this, but it remains to be seen whether that is true in practice.
- The wires south of Bedford require upgrades to operate above their current speed limit (I forget if it's 100 or 110mph max, but it dates back to the Bedpan upgrade days when 125mph operation anywhere on the MML was a pipedream and 319s were the benchmark for wire calibration on what is in effect a closed system). Network Rail would have to really pull their finger out in order to deliver this in time for their introduction. In theory that shouldn't be an issue thanks to the superior performance of an IEP on electric power, but given they're almost exclusively direct south of Bedford anyway, any time gains from acceleration are going to be minimal and lost in those (admittedly limited) stretches where they cannot operate at the 125mph allowed of diesel-powered MML trains.
The second issue has come up a few times in public consultation meetings I've attended through work, and to be frank NR have no answer for it at present unless I've missed something . Simply put, it's unlikely that anything will match the timetable performance of a Meridian, which the 2019 timetable is based around, on the full route without upgrading the route south of Bedford to the same standard as the new wires up to Market Harborough.
For that reason, it concerns me that there's talk of wholesale fleet replacement including the Meridians, rather than targetting the HSTs at first and seeing how their replacements perform before jumping the gun and potentially reversing the positives of the new timetable.