I believe the performance hit depends a lot on how much data is being changed between snapshots, and when/how often your snapshots are scheduled to run.
Shadow-copy generally doesn't do anything to your read-performance instead what happens is that whenever you write to a file that is considered to be part of a snapshot a copy will be made and your write will be made to the copy.
On one system I was responsible for I enabled shadow copy on our server, given your description I may have a roughly equivalent situation to yours and it didn't really seem cause much of a performance hit at all. I have about 100 users and around 600gb of data that is most doc, pdfs, xls and so on. I do 2 snapshots a day.
I wish I could offer you something more concrete, but since it is so dependent on your specific usage patterns, you may have to warn your users and simply try it. You probably would want to start with a conservative one snapshot a day schedule, and then increase the frequency as needed/desired.
It will be important to get some performance data on the server before you make any changes so you will have some data see what impact the change had.