If you open the ChromeOS Task Manager (use the Search + Escape keyboard shortcut) you can see how much system resource is being used by each process on the machine, including extensions.
I tend not to run a lot of extensions, and I usually just enable the ones I need when I need them (using Extensity, highly recommended), not for performance, but because I don't like clutter. But even when I do run a lot of extensions I don't really notice a performance hit.
That said, It might depend a bit on the Chromebook you're using. All my Chromebooks have at least 8GB or RAM, so it's usually not an issue, but if you're pushing a bunch of resource hungry extensions to a machine with only 4GB, it's possible you might feel it a bit. Generally, it should not be an issue, but use the Task Manager to try and quantify the impact.
From what I can see, the extensions do take up RAM, but don't seem to impact on CPU load.
Ultimately, try it yourself as a user, using the target Chromebooks you plan to push them to, to see if you notice anything. If you can't really tell the difference, it's probably going to be fine.
Here's a couple of articles you might want to check out for further reading...
