Yes, they 'stick out a bit more' precisely because the GW loading gauge allows a 23m coach with a wider body shell than the 'go anywhere' Mk3. But, like I said, it is still nowhere near being on a par with continental loading gauge and it never will be, which is what you would need to have a 3m-wide body to fit in your 'comfortable' 3+2 seating - not that anyone seems keen on building stock with 3+2 seats anyway - LM are reported not to be that thrilled with their recent batch of 3+2 350s, compared with 2+2 versions of the same type, or their 172s.
And I suspect that were new stock to be ordered for GW outer-suburban electrics, though this seems unlikely, then operators would probably go again for a 23m body shell, with the couple of extra rows of seats that would allow. And leasing companies would not be that keen to have short, fat coaches that couldn't go anywhere else on their hands, in case they ever need to shift them on to other work.
It's not all about the body width anyway - you need to take account of things like step boards at the doors, and crew steps on bogies etc. These can also hit things, which is why Turbos that go 'off-piste' for work at places like Ilford have the step boards removed first.
We are stuck with what the Victorians built and short of replacing thousands of bridges and tunnels, rebuilding platforms, etc, etc, at obscene expense when there are better things to spend the money on, there's nothing we can do about it.
IEP isn't all that bespoke, it's based on trains Hitachi builds for the 3ft 6in gauge network in Japan.
