Cobber,
Probably would have made barrel clean up easier if you'da done the "proper" barrel break-in. But, since you're past that now no need to worry. I italicized proper because there could be another whole post on that subject.
I have noticed that whenever I use a brush on any of my barrels, no matter what stage of clean they're in, the patches always come out black. Just the nature of the beast I guess. I use the brush to start with and jag and patches from there on out.
I use Barnes CR-10 to remove copper and carbon now. It, like some other solvents, has a high ammonia content and the directions on the bottle should be followed to the letter. I use this until patches stop turning blue/green on me then go to Kroil/Marvel Mystery Oil combo, and dry patch out.
Sweets and Montana Extreme are two more "odifirous" (read: ammonia) solvents that work on the copper. Never have used JB bore cleaner or paste, but a gunsmith friend swears by them. Uses them after initial cleaning to smooth barrels out. I'm not sure if the abrasiveness of this is too much for a barrel, it doesn't seem to hurt my buddies barrels. I would think that after 400 rds your barrel would have most of the roughness and tooling marks out. Especially with a hot little number like the .270 WSM. I have been considering that round myself.
As for the Tubbs firelapping kit, I won't say either way. I have no experience with it either. I've never ran across a barrel that was so rough I couldn't get it smooth by other means. It too is probably cause for another post.
I have a Sendero 7mm RM that the first 10-12" of bore is really rough (1500+ rounds!). Instead of continueing to waste 2-3 days to clean I am planning on an aftermarket barrel for it in the near future. Good luck with your barrel and let us know what you do. Johnny K.