He has to be. Bandon was chosen as apprentice to Malak from his various followers and acolytes. Sion was chosen out of the best Sith at the Academy, the ones who passed all the trials and tests put forth by Traya. And like I said, he was right next to Nihilus under Traya, and was one of the two leaders of the Sith forces after his Master's exile. Plus, just think about it. It just seems logical. I'm not saying that Sion would own Bandon. But he's certainly capable of defeating him.
genises, this isn't twenty questions, lol. But that Council member, having lived through two wars, was probably pretty good. Now speculation? Of course. But see Maul only killed two people worthy of notice, and although it is due to his arrogance, he got chopped in two by a padawan. That padawan Obi-Wan would become in ROTS, far better than Maul. And Sion would give him a fight, if not kill him. I'd say Sion was under Dooku, but above Maul for sure.
*sigh* The fact is, Maul didn't live. This isn't "Maul if he'd lived to ROTS." This is TPM Maul. Don't complicate things, lol. Now, a Maul who'd survived 'till ROTS would be a force to be reckoned with, for sure. he'd probably be able to beat Obi-Wan, Anakin, any of the Council members save for Mace and Yoda, who'd still take him, and many others. But TPM Maul? he'd going down. Plain and simple.