First point, SP speeds allow the 170s to run at a higher speed than other stock. There's a fair bit of SP speeds east of Leicester. For example, between Peterborough and Stamford in the down direction there is a section which is SP 75 but other stuff is limited to 50mph around Bainton. Over in the fens there are bigger differentials, especially the long section from Ely North Junction to Lakenheath (45 vs 75mph), though this isn't on the Stansted route.
This is where I don't understand why older stock seems to have some sort of advantage over more modern units, which I would assume are lighter / weight distribution is better.
Perhaps it's a matter of physics, or just a matter of effort - that nobody has really bothered to clear Voyagers / Meridians of higher speeds as it's not usual for them to take that route.
However, as I posted before, a bit of shifting stock here and there and XC could operate with 3-car trains all the time:
While not a great upgrade, the 3-coach 175s from TfW could be transferred to XC (16 175s to replace 13 2-coach 170s, giving three extra units to play with) so there's no reason why all the Stansted trains couldn't always be 3-coaches.
Then, send the ex-XC 2-coach 170s to EMT, and together with the all 170s from WMT, they could replace the whole 153/156 fleet over there as the numbers of units match exactly.
Sending more 2-coach trains (from WMT) to XC would only mean doubled up units, which they're not going to want to do as that means no revenue from one unit (which may annoy the SC), or double the staff, which clearly isn't an option.