Having been on a Class 395 since the post that Royston Vasey made in the thread
http://www.railforums.co.uk/showthread.php?t=72620 the ambience of these trains is good but is not like travelling ona an HST either back in 1976 when they where introduced or like they are today with FGW which I also travelled on recently to Worcester Shrub Hill.
There was more space between the seats in the HST than in the class 395, which seems to be less space as it is going more for the commuter market than those that are travelling not just for leisure, but are travelling inter city between the likes of London, Reading, Bath, Bristol, Worcestor etc.... This is where Hitachi must with the class 800/801 trains have seating the same as the HST has today.
Getting back on subject of this thread, as was stated within the first few posts of this threads, you will always have clearance issues with any new train. You had it with the HST and MK 3 coaches, there was clearance issues with APT on the WCML from memory to do with space from the doors to the platform.
There has been clearance issues with more recent trains as well, Voyagers, Meridians, Pendolinos and Eurostar trains when used by GNER which could not travel further North if my memory is correct than Leeds due to the articulated nature of the Eurostar carriages.
Since the Eurostar trains where only in temporary use, there was not need to get clearance north of Leeds. But I think whatever the train the issues of clearance as has been stated before, would be a) factored into the budget of the trains and b) would have been thought about long a ago such that work has either already been done or is planned to be done during a feature engineering work time, such as the work on rebuilding Reading station where factors where put in place not only for the existing trains, but for the class 800/801 IEP trains as well.
What also helps in the scenario of the IEP trains, is the fact that we already have the class 395 javelins running on both HS1 and domestic routes as it is 90 - 95% the same train that the IEP trains will be, just with shorter carriages.