I would state that Maul is pretty powerful. It is most likely inarguable that Maul was one of the best martial artists in the entire Sith Order. As stated, he was a master of Teras Kasi, the best martial art form against force-users mostly due to its usage of mental barriers. Maul was also pretty good with Juyo, and Niman, and from what we see in the movies, he is reasonably competent in Ataru due to his usage of acrobatics, and was able to defeat Qui-Gon, a powerful Ataru master(who had somewhat bad stamina).
When we look at Maul's fight with Savage against Kenobi, and Adi-Gala, Maul was able to -at the end- pull a starfighter using telekniesis, and pull it down toward the ground. While, in that era, there were most likely a lot of Jedi Masters who could probably perform a skil like that, it does show that Maul is a master of telekinisis, and is pretty skilled at it. Maul is also pretty good with light-saber combat, exemplified by the fact that Savage Opress was able to beat Adi-Galia in combat, and Savage was just a guy with brute force; Maul was able to beat Savage in the beginning parts of that episode specifically.
I've also looked at Maul's fight with Sidious(after Savage was killed), and Sidious fighting 3 Jedi Council members, and Maul lasted a lot longer than all of them(seconds apart, but still longer) except with Windu, who was an absolute powerhouse in lightsaber combat.
In terms of his capability to overthrow Sidious, no one execpt Anakin, or Luke can be able to do that tbh. Sidious was probably the most powerful Dark Lord in history, and Maul was just an Assassin, he wasn't a true Sith apprentice(something that I believe Sidious himself even mentioned).
I hope this answered your question.