Right now our site has a Contacts page with a list of contact profile names which link to a separate profile page. My team suggested we design an accordion style (expand/ collapse) list for our contact profiles. Is there a downside to keeping editable data like name, phone, email, etc within an accordion list of up to 300 contacts?
I think the bigger issue is the fact that you have a single list off 300 contacts.
First, I'd ask why you need so many. If there is a valid reason, can it be segmented further? Departments? Classes etc? If so, do that and have multiple pages with the segmented contact info
Accordion style UI elements have always bugged me. You'll run into issues on mobile devices when the user clicks to load a list of 300 contacts. Some people simply miss the fact that there is a "button" to push and don't see it at all.
Don't hide the info any more than you need to. Get it in front of the user as soon as you can.
I think it depends on where and how you would like to use it. If you intend to make it mobile friendly, then older or low-end cellphone might struggle drawing and animating 300 collapsible panels, and scrolling would be very painfull.
Personally I would go with a simple list (see https://www.google.com/design/spec/components/menus.html#menus-usage), do an ajax-request if someone clicks on it and then append phone, email etc. below the name.
If you know it's just for modern desktop computers then collapsible panels shouldn't be an issue.