Although belief in tahrif of the Qur'an is non-standard, it is better to reserve the word kufr for the things that Allah directly uses it for.

The Qur'an does not directly say that belief in tahrif is kufr. (Rather, some people assert this based on a deduction/interpretation of a verse, not a direct statement.)

The word kufr tends to be thrown around quite casually these days. There is rarely any benefit to accusing people of kufr or labelling Muslims as kafir just because one thinks they are incorrect in their belief. It is quite sad that this intolerance has developed today. The classical Islamic era seems to have been much more tolerant and open to discussing differences of opinion regarding theology. This is not to say that every view was always right, but it is better to be able to discuss things rationally rather than to shut down dissenting views by just labelling and attacking (verbally or sometimes physically).