AIUI the 1000V rating on test equipment basically means "this is designed to survive the spiciest transient you're likely to encounter on 'low voltage'[1] installations unless something Extremely Wrong happens in the distribution network".
Meanwhile a 300V rating means "if you connect this to the mains, it may result in incorrect readings, magic smoke release and even DETH". Note that 240V AC peaks at around 339V, and if you don't understand why, you shouldn't IMHO be connecting multimeters to mains wiring.
On that basis, I wouldn't get too upset that a '300V' rated meter fails to give a sensible indication of overload voltages above that rating, on the basis that it's only really suitable for continuity testing, battery checking, hobby electronics sort of things.
A sensible rule of thumb is you want a proper Cat III rated meter from a reputable manufacturer for working on live mains. It's about how it fails when exposed to high voltages as much as whether it can measure them.
[1] In the distribution sense, so 240V/415V AC mains.