There are several reasons I don’t think 300+ rows in a repeater are a good idea.
One of those reasons is management. The admin for this repeater will be difficult to use.
Another reason is the potential for timeouts and other errors when saving.
PHP has a limit on the number of input variables. It can be adjusted, but if you go beyond this there is not warning, you simply see rows disappear, unless you have a script in place that gives a warning when the number is exceeded.
Another potential issue is timeouts when saving. Depending on the data in those rows there is the potential that saving will fail because it takes longer than 30 seconds.
Also, if I have a repeater that will likely have this number of rows then I’m probably not using the right tool for the job. More than likely the data would be better served by using a custom post type or a custom taxonomy, depending on what data needs to be stored. This may not always be the case, but I would explore other options to make sure that a repeater was the right choice.
