@The-Engineer-That-Brews
I tried the cherry flavourings in samples of the porter base and didn't like it at all - too synthetic tasting.
In the end I went with the Lowicz cherry cordial and used two bottles in a single 19L corny.
I would suggest adding the cordial to the fermentor in secondary. Because I didn't discover that my use of cherry puree was a complete waste of time (in the sense it didn't add any discernible cherry flavour) until the beer was kegged, I had to add the cordial to the keg, which caused a secondary fermentation due to the sugars in the cordial and massive overcarbonation.
That said, the cordial was successful in imparting a palatable cherry flavour to the beer. I wouldn't say the flavour was dramatic - very much at the subtle end of the scale, like a mild marzipan.
I have no idea how they get strong cherry flavour in big beers. If you've ever had the Vocation black forest imperial stout, you will know what I mean. I can only assume it's massive quantities of fruit, or some other black magic I'm oblivious to.