That Des Moines supply house guy is giving you half the story. I run a 6 inch brush on 6 inch round liners all the time and yeah its snug but thats the point. If the brush is too small you get slop, and slop means you go back down a second time which kills your arms worse than one slow pass. The real thing nobody talks about is the rod, not the brush. A stiff 6 inch brush on a cheap flexible rod will wreck your wrists because every bit of drag goes straight into your hands, but put the same brush on a rigid rod and it glides fine. Ive done hundreds of 8x8 flues and the guys I see burning out are usually fighting the rod, not the bristles. Going down to a 5 inch just means you leave more behind and you find that out on the next sweep. Check your rod before you blame the brush size.

That line about foil steaming the bark right off is exactly what happened to me. I ran foil for years and could never figure out why my bark turned to mush by the time it hit 205, then a buddy told me the same thing and it clicked. Switched to paper and the crust finally had some bite to it, that part is not even close. The longer cook time is real though, mine went up right around an hour and a half too, and losing that juice means I have to spritz more than I ever did with foil. Where I landed is paper for butts I plan to slice or serve on a plate and foil when I want that soft, wet texture for sandwiches with sauce. Going on two years now and I have not gone back to foil full time.