Palpatine himself is interesting to discuss in light of the Rule of Two, because it's questionable how strongly he followed it. After all he took Darth Maul as an apprentice before he killed his own master and began his training in the ways of the Sith. Granted, Zannah did something similar to Set Harth, but she never named him a Darth or truly trained him. Also, didn't Sidious kill Plageious in his sleep? I'm not sure if Bane would consider that in the spirit of the Rule of Two. He said the title had to be "wrenched from the all powerful grasp of the Master." Of course you could argue that if Plaeguis was trusting enough to let Sidous kill him in his sleep then he was too weak to lead the Sith, but it's still debatable.
also, Plageious killed his master two in an underhanded way (cave in), not to mention that he was grooming another apprentice to replace Plageious. it's mentioned in that book that not many after Bane and Zannah strictly followed the rule of 2. i agree that Bane would have wiped the floor with Sidious and probably Vader, but Bane's solution came from l\Lord Kaan's "brotherhood" idea. it was (rule of 2) wise at the time, but didn't fully pan out in the end (Vader killing Sidious, then basically being killed in the process).