I use the method you said you weren't too interested in, thought I'd comment anyway.
With the cold extraction coffee method I use french roast beans, ground then extracted as per the normal process. This is what I drink every day, not just a beer thing. I get about a quart of a pretty strong extract using a Coffee Toddy. Here's a pic of it http://www.toddycafe.com/shop/product.php?productId=67
(I like it, down't own stock, it's a good tool). There's probably a million ways to do cold coffee extract, the Toddy is convenient.
I make a brew I call a "Mocha Stout", basically an Oatmeal Stout with 1 pound Lactose, 2 pound bakers chocolate, and one quart of my extract. It's a pretty robust brew, actually a good party beer since it's so sweet, even the Women will drink it. It is a 10 gallon batch.
But the point is that the quart of extract gives the brew a very obvious coffee flavor. The chocolate is right in your face also but there is no mistaking the coffee flavor. In a regular stout or a Porter that quantity of extract would be too much, unless you wanted a "coffee porter" or something. Works well in an Imperial Stout also, but not the entire quart.
For your porter, a cup (in 5 gallon carbouy) would be a pretty good estimate. This assumes a good robust extract like you would get from fresh ground quality french roast beans. A lesser extract and...you get the idea.
If you preferred a more subtle coffee presence then a half cup in the carbouy would be a good start. This would be more of a coffee as complexity addition rather than coffee as an identifiable flavor component.
There are (supposed) benefits to the cold extraction system, namely less acid and fat. True or not? Who knows, it's good coffee to drink and I have good results in my brews. Never a head retention issue, etc.
My opinion in the choice between extract and beans is you will achieve a much more precise and replicatable end product with extract than with beans. You might get the first experiment wrong but once you know the actual amount to add that you like, that will be unlikely to change for future batches.
Also, I add my extract just as I'm turning off the heat for the boil, what would typically be the aroma hop stage. Get a little sterilization thing going that way. Doesn't seem to affect the coffee aroma or flavor in the end product.