The only disadvantage to leaving a 6 or 7 bone roast whole is you only get two end cuts, and some people really favor an end cut. Once you get over 11 or 12 pounds the length/diameter works in your favor and cook time moves closer together. For example, lets say you had an average 8" long pork loin roast and it reaches 145° in 90 minutes. Cooking a 15" or 18" long loin roast will take about the same amount of time because the diameter is about the same, it just is longer. Keep in mind that 'shape' on a rib roast will vary from roast to roast more than my example of a pork loin... so always monitor with a thermometer.
I use a maximum of 250° for my cook times and below are some times from a database of prime rib cooks you could use as a gusstimate for your big day. Don't forget carryover temp, because a large roast will gain a few degrees while resting. To play it safe, don't schedule an absolute sit down time and to really play it safe, while the roast rests, warm up a cast iron skillet on your cooker. If anyone's slice looks too rare for them, zap that slice for a few seconds on each side and serve with the most 'done looking' side up. This fools the eyes into thinking it's more cooked than it really is. Once they bite, they will probably love it. All that said, after a couple of hours if you notice your internal temp rising a little too fast, ramp down your cooker so the internal temp will ease up to your preferred doneness.
10 pound roast - 220°-228° pit temp - 3 hours to reach 120°
11 pound roast (4 bones) - 215° average pit temp - 4 hours 54 minutes to reach 125°
14 pound roast - 220°-250° pit temp - 4 hours 30 minutes to reach 125°
15 pound roast - 220°-250° pit temp - 4 hrs 50 min to reach 127°
19.5 pound roast (prime grade) - 240° pit temp (dome temp) - 4 hours 25 minutes to reach 124°