Thanks for the responses.
The reason I asked the question was that I recall reading, around the time the "networker express" units were created, that the selected units would be regeared to 90mph for timetabling purposes as well as having first class added etc.
Interesting that this has not been done. As others have said I wonder whether it's a combination of better acceleration offsetting the top speed deficit (how much 75mph+ running do 375s really achieve, particularly on semi-fast services?) and lots of slack in the timetable following introduction of the javelin services.
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Also, I understand 465s can be coupled with class 365s (100mph) and have a similar power output. Presumably regearing 465s to 90 or 100mph would be theoretically possible?