Syphon shouldn’t be adding significant overhead – the parts of the process to do with generating and affecting your content are likely to be the drain on resources. Only start to worry about it if you are starting to hit limits (eg dropped frames). In that case, try each part of your pipeline independently to identify the part that is struggling. If you still see problems without Syphon being involved, then Syphon isn’t the problem.
There is no simple maths you can do. Syphon treats all content the same – the only significant hit Syphon will cause is when you resize the Syphon frames, in which case the underlying surface is rebuilt. Otherwise the only limitation is, as with any graphics work, that the more pixels you are working with the more work needs to be done.