displaying a webform in an overlay
If I need to display webform in popup, then I would use ctools modal.

You can use modal forms module which make use of the modal feature in the ctools module to open some common forms in a modal window.

Supported forms


Log in (modal_forms/nojs/login)
Request new password (modal_forms/nojs/password)
Create new account (modal_forms/nojs/register)
Contact (modal_forms/nojs/contact)
Comment (modal_forms/nojs/comment/reply/%node)
Webform (modal_forms/nojs/webform/%node)


May be this is not answer to your question as you are using colorbox, but I am adding this so that you can check other option.