This already poses a deep ethical dilemma. To add a gene to an animal to induce cancer could never be justified if it were not for the seriousness of the human disease. If this is so for mice, how much more so for primates. It is hard to see what would ever justify genetically modifying monkeys to model human disease. Most people value large mammals and primates higher than small mammals like mice and rats, because of more human-like characteristics.
Primates possess much higher levels of sentience, consciousness and socialisation.
It is not enough to argue in this case that we need to use monkeys because they model human disease better. The more exactly the animals model the human disease, the more they are likely to suffer. Whatever animal is used, a fundamental gulf always exists between disease in humans and in another species. Even GM primates would still leave that gulf unbridged. In 1995 the UK Government's Banner Committee proposed ethical guidelines for assessing novel animal technologies, which have been widely accepted.