I don’t necessarily agree that lack of back pressure will burn exhaust valves. If that were the case many an airplane would have fallen out of the sky due to short exhaust stubs.
In fact, one of the methods of IDing an aircraft at night during wartime was to note the exhaust flame patterns.
Running short, unbaffled exhaust pipes on motorcycles also do not cause a problem.
Since this is not a problem with inherently hotter running air-cooled engines it should not be a problem on liquid cooled automobile engines. This is proven by the guys who run modded cars on the street with a straight exhaust; as in no mufflers or converters. A friend of my son ran his Mustang 5.0 like that for years with no problem.
I’d say if someone is burning valves they have a mechanical lifter holding an exhaust valve open a half-thousandth of an inch, they’re running too lean, or something to that effect.