I would argue that the 5-car IETs are as close to being a good regional express design as they are to being a good InterCity train (ie. about half way between the two, and not a particularly good design for either). The 158s and 175s on the other hand are near-perfect* for regional express, if operated in multiple on the busy sections (which, in GWR's case, is probably the whole route but portion working can be useful elsewhere (eg. the Cambrian lines)). It's not just GWR that doesn't have a good 5-car regional express design but the whole of GB - the closest we have come on this island are the class 444 and 442 EMUs.The real problem is that GWR mainly has the express IETs and suburban units, and nothing that even half resembles a 4 or 5car regional unit. The 158s and 175s are all short, not suitable for busy regional routes unless operated in multiple.
* given their build dates and remaining life; these days any new-build stock should be electrification-ready. Otherwise, the 158/159s are nearer to the mark in terms of running costs and provision of end gangways, while the 175s have an interior and window size (allowing both decent legroom and decent alignment, apparently, while the 158/159s only allow the latter) which comes closer to the optimum for this role.
