I don't flip or baste. 6 hours seems way too fast for a tender sliced brisket...are your smoker therms accurate? Is that a large center-cut or a very small whole brisket (packer)? At that pace, I would think you're pushing closer to 275*, but then again, a flat (center-cut) will cook faster without the point attached. I rarely smoke flats, but it seems like I run 225* for the first couple hours, then back it off to around 205-210* to slow it down for better tenderization during cooking.
Also, 200* will be a dry pull unless the fat-cap is still attached. 180-190 for slicing. Don't forget to rest it, foiled and wrapped in towels for an hour or so.
Eric