In that case, your users won't suffer from it. They will do their day-to-day backups to that "local" repository and the backup copy job will not interfere with that, at all. Now it is difficult to guess what your ratio will be, but I ask you to don't believe me and test out for yourself (maybe with your own PC) and do this (make sure you use the populate cache feature!). The results will be fantastic (did it myself and got 36x which is pretty cool in my book). And the results will only get better when there are more PCs (think about it, a lot of your PC backups will be exactly the same...).
I know it isn't seeding, but you will be surprised by the results. But again, don't believe me... See for yourself