Can I filter on other properties than the project id in getData?
No, the only filter criteria that you can specify in the getData method are the project id and which optional properties to get. The idea is that you parse the JSON at your end, and then traverse the resulting data structure and filter out the data you need. This way, you get a consistent snapshot of all of your data, and you can filter out exactly what you need, at your end.
Don't worry about the amount of data that gets transferred. Our servers are optimized to efficiently provide large amounts of data from getData. But you should, of course, only retrieve the projects that you need, and only include the optional properties that you need.