There is a great discussion about using honey going on in this thread:
Any tips on using honey?
The biggest contribution from honey will be sugars. Any sugar acts to thin the body, produce more alcohol and dry out a beer. Not all bad things, but possibly not the best for a porter. One poster suggested adding in the honey when you have the wort cooled to around 150 to preserve honey aroma.
A good experiment would be to get some good mead and blend it with the beer you intend to make with honey. This might give you some indication on how the actual beer would turn out.