Will a diploma thesis written with a declared use of GenAI tools within the per-missible extent be graded differently (worse) than a thesis written without using such tools?

No, if the use of GenAI tools was agreed with the supervisor and did not violate the rules established by the University. The grade in such a situation will be the same as in the case of a thesis written without using GenAI tools. Remember: research shows that excessive reliance on GenAI assistance weakens critical thinking and problem-solving skills, and the resulting papers are characterized by lower quality than those created without or with little artificial intelligence interference.