5-car IEP is 315 seats, so slightly more than half an IC125 but taking a class 175 as a 'conventional DMU' you'd get 310 seats in a 5-car formation (one 2-car set and one 3-car set). If you made a 5-car 175 with three intermediate cars and two driving vehicles it'd be 325 seats. So the extra length of the carriages in an IEP is roughly speaking enough to compensate for the kitchen and 125mph crumple zones in a 5-car set.
The route through to Penzance was always included on the map of IEP route clearance, even when IC125s were to remain the main fleet for that route with IEP ending at Exeter. I seem to recall the cost estimate for the work was in the hundreds of millions (but not billions). However, Pembroke Dock was not included on the map, I'm not sure if that's because DfT forgot it (given that it is a minor issue as far as the GW franchise is concerned, although very important locally) or whether they assessed it and found Narberth tunnel was too tight (in which case I wouldn't be supprised if the cost of clearing that route could be almost as much as the rest of the GW IEP clearance programme put together).
I am extremely concerned that most of the 91s won't find work once IEP has arrived in about 2020, which in my view would be a terrible waste. More 180s (perhaps even all of them) are supposedly going to end up with Grand Central but I do wonder whether the prediction (I think by one of the Modern Railways coloumnists) that there will be Voyagers cut up before the last Pacer is gone is still possible. My guess is, if that does happen it will probably not be whole sets, but driving vehicles with the intermediate trailers used to lengthen other sets (VT has had a basically useless 2-car set for some time, if other franchises do similar some might be scrapped).
But what about the busy summer weekends in south Wales? And Cardiff-Swansea all the time?
I believe bidders for TPE are permitted to use 442s in some shape or form. Whether one of the bidders is proposing loco-haulage of them I do not know.
One could very well say that a high-capacity HST has
slightly more seats than the HSTs did when first built - so what? And part of the extra length in an IEP is providing cycle stowage and extra luggage space on what is designed to be an intercity train, so not sure what your point about a 175 is, when we already have a five-car intercity train to make comparisons with in the shape of the Class 180, built using the same basic bodyshell as the 175, which seats 280 people and has a whole coach of first class, a feature which appears to be missing from the 175s.
I know you're obsessed with Pembroke Dock but one train each way for a few summer Saturdays is, as you say, a minor issue in the context of the GW franchise, especially when that train is provided in large part to help meet local demand, not for travel from England to Tenby, so not hard to suggest that in the long run a local solution to South Wales's summer Saturday needs should be found through the Wales franchise.
Similarly on the South Wales main line, I think there is every chance that acquisition of emus will allow a beefing up of the ATW or whoever else it is by then's semi-fast service in three years' time - indeed it would probably be downright odd if the opportunity to provide more capacity that way was not taken.
And Cardiff-Swansea does not need an hourly 630-plus seat express train 'all the time', though given the location of the new depot at Maliphant, it will probably get a few more of them at either end of the day than it would have done if Hitachi had chosen a site in Cardiff instead.