I've heard tell from an East Coast driver that the 91s often have difficulty in getting away from Durham against the gradient (southbound) with the current formations as they are: The 91s have got plenty of power, but difficulty in applying it to the rail because of the limitations of the Bo-bo wheel arrangement. Adding an extra couple of carriages to the formation (and as has already been said the IC225s were originally intended to be ten carriages in length) would probably exacerbate this problem but as you say regearing the 91s for a 125mph maximum would do wonders in alleviating it.
GNER I think it was had the idea of reforming the mark 4 sets into 10 carriage rakes and then forming up three rakes of mark 3s to make up the numbers after three mark 4 sets had been disbanded to allow an extra vehicle to be inserted into each of the remaining 27 sets, to work with the existing class 91 fleet, so 125mph would still be achieved and no new electric locos would be required.
When the production HSTs were being designed there was apparently a 3000hp power unit that would have fit into the power car bodyshell, but this idea was never taken up as the additional power was not necessary and would have led to higher fuel consumption and there were also concerns over higher axle rates. And of course, the East Coast HST sets were originally meant to be higher rated at 2500hp, hence them being class 254, to cope with the longer eight carriage formations although nowadays they cope perfectly happily with nine carriage formations still with 2250hp per power car.
