I poured half a glass into identical glasses and added oak essence to one and heavily oaked (its as black as RIS) bourbon to the other.
Oak essence:
I was surprise to find that I really liked the oak flavor in this beer. It was very subtle, only1 2 drops of the essence to half an imperial pint. I added another drop and swirled it in. I liked it a little better, but the oak was perhaps a little too pronounced. Added another drop, now the oak was overpowering the rest of the flavors. 3 drops was too much. If there was such a thing as 1.75 that would be perfect I think.
6oz medium plus oak cubes covered in Jim Beam black label for 1 month:
This stuff isn't as concentrated as the oak essence, so I added two drops at a time. I could pick up the bourbon in the aroma, and in the finish right from the start. It's not bad, but I don't think it's better, just different. I added another two drops and now I pick up the bourbon right away. It fights with the graham cracker flavor. The flavors don't compliment each other at all. I add another 2 drops and it just gets worse. Lots of bourbon just doesn't belong in this beer, and least not this bourbon.
The Jim Beams Black isn't bad bourbon, but it isn't very sweet either, so just to make sure I added oak essence and some Fighting Cock to another half glass. This is quite different. Fighting Cock is very heavy on the caramel and is very sweet. It works much better with the vanilla and chocolate in this beer. This one I actually like quite a bit. The caramel from the bourbon and the vanilla from the beer and the oak play against each other very nicely. I really like this one.
Well crap. Now I feel like my testing wasn't worth much to you. To sum it up, this is what I found:
Oak alone is quite tasty in this beer
Jim Beam doesn't work with this beer at all
Fighting Cock married quite well with the beer if used in small quantities
So I guess it comes down to what your barrels had in them. It also depends on if they have been used for other beers. If they are fresh from the distillery then the type of bourbon that was in them is going to make all the difference with a beer like this. If we were talking about an RIS here then I would say go for it, but with this beer I think you need to take the character of the bourbon in those barrels into strong consideration before you go forward. I hope I was at least some help to you. My recommendation would be to pick up some of the bourbon that your barrels held (or use some of what is still in them) and try a bit mixed into the beer before you fill the barrels. If you like it then go for it, but if you don't you could save yourself some time and heartache.
Best of luck to you, and please let us know how it turns out. I will be very interested to hear your results!
*Footnote*
After brewing all day (I usually don't eat or drink anything at all so that I am now dehydrated and have an empty stomach) 1.5 pints of this beer did me right in. I think it's time for bed!