The legacy of the broad gauge was that it left behind structures, notably bridges, and tunnels with much greater lateral clearances, so you could run wide-bodied standard gauge stock, but GWR main lines generally had a more generous loading gauge than other railways - including those the GWR built after the end of the broad gauge, such as the GW&GC Joint and the Bicester cut-off. That's why Turbos can use them, with bodywork that would be out of gauge in many other places. The Super Saloons were even wider because they are shorter, so don't have as much overhang in curves. The constraint on other railways using wide-bodied stock was structure-gauge issues, not track spacing.
As I said, at Moreton-in-Marsh station you can indeed see a wider than usual track spacing but just along the line at Kingham and Evesham, the GWR rebuilt the platforms in the1880s and they were positioned to normal standard gauge spacing. That's how far back the process of moving track position goes.
Yes, there are places where wider than usual plain line spacing still applies - I said that - but the majority? I don't think so. Track renewals over many years now have been carried out to usual rail industry standards, including pretty much every inch of the new double track on the Cotswold Line laid in 2011. The extra width on the trackbed there was used to provide a trackside walking route for engineering staff.
