Have you seen a video of an IET and HST leaving the same station. I think that you will find that the IET is actually quicker to acceleration speed than an HST. A good example of this is the video
uploaded 5 years ago. It shows 8-car HST leaving the same station as 10-car IET. It maybe as you say that the IET trains spend more time at full power with their engines, but with that the IET would in fact be up to linespeed quicker than the HST's.
Yes, the HST is the more comfortable train, but the IET sets are quicker to accelerate.
XC certainly does need more stock and needs to hand over more 'local' routes to the operators such as Nottingham to Birmingham New Street to WMT and Stansted Airport to Cambridge to Greater Anglia.
The most ideal situation would be the majority Avanti class 221's going to XC, with a few to Grand Central.