IMO, the best coffee flavor will come from the best brewed coffee. My personal preference when drinking coffee is to go for a well-brewed espresso. I've found that adding espresso at kegging will give you the cleanest coffee flavor and also afford you total control over the dosing rate. I've tried several cold-brewed coffees this last year and they were all remarkably bland and muted. The reasons why some laud cold brewing are the same reasons why I don't like it. You miss out on many of the assertively flavorful compounds that are only extractable at higher temps. I'm unsure of the extraction chemistry with espresso, but the high-pressure super-heated steam definitely makes for a very uniquely bold cup, even when watered down to drip-brewed strength, aka "Americano."
There are also several reasons not to add coffee in the boil kettle:
1) Any temps much beyond 180-190 will extract nasty flavor compounds from the grounds - akin to sparging too hot.
2) In dark beers, such as stouts and porters, the pH of the wort is already going to be pretty damn acidic. The yeast don't particularly like the pH to drop even further.
You need a good espresso machine anyway and now you have an excuse. Beer doesn't have to be the only brewed beverage in your house