As it happens, I just bottled a Coffee Porter last night. I sampled the leftovers, and it had a very strong coffee flavour.
I brewed it as a 5 gallon batch, then after 2 weeks of fermenting, I racked it to 4 one gallon jugs (and bottled the remainder plain) and added different flavourings to each jug (vanilla, gingerbread, pumpkin spice, and coffee). The coffee one got 40 grams of espresso grounds (from the grocery store, in a can), just dumped directly in. I let it sit for a week, then cold-crashed it for a night and bottled it last night.
The coffee one had a strong coffee aroma and flavour. It still has to carb up, and I don't know if the flavour will be as strong after chilling, but so far it's definitely a "coffee porter," without being overpowering. I think I lucked out on the amount of coffee (40 grams espresso grounds dry hopped in 1 gallon of porter).