Seems to be a mixture. I recall them being modified to be able to rescue 377s etc, having been a different height previously, 390s having them at "non-standard height"
as i have posted on another thread... im 99% sure that the different coupler heights between the pendolino/22X units and other dellner like couplers is down to EU directives for high speed train interoperability/specifications.
We already know that Pendolinos and Voyagers can couple together (atleast mechanically).. both of these falling into the high speed trains catagory.
So that should mean that a Class 390 Pendolino could couple upto a DB ICE, SNCF TGV, a Eurostar class 373 or a SouthEastern class 395
other units ie. turbostars, desiros, Junipers and Electrostars would not be covered by the EU directive/specification as they are not classed as "High Speed Trains"
and since the 57/3s were modified to allow use with pendolinos... thats why the couplers are (iirc) set higher, than if they had been modified for use with desiros or electrostars.