Well given that he already lost to Sidious..... Quite handily, in fact.
I will point out again that he lost in unfair and onesided contest. He had no absorb ability or lightsaber to defend against lightning.
Neither canon source shows him losing "quite handily".
Bane's level of swordsmanship by DoE is enough to overcome whatever Starkiller brings his way.
Marek drilled his saber skills just like Force skills from childhood. And he demonstrated his skill on multiple occasions and nowhere he was shown to be inferior in any way.
Whether Bane or Marek is better cannot be determined.
As stated, his main chance would be in gathering an immense amount of power.
You think so? Where exactly is it stated that he needs to gather power?
How about this:
"He caught the general in a choke hold and maintained his grip even though it turned partially back on him. He had been ready for this; his lungs were full. The general, however, clutched at his throat with one hand while barely managing to parry with the other. The apprentice let the fire in his lungs fuel his lust for triumph. Even as darkness crowded around the edges of his vision, he sent objects hurtling at Kota's legs and face, battling him on all fronts.
Finally a fragment of smoking debris struck the general's knees from behind. With a cry of frustration, the flailing Jedi went down, his face purple and eyes bulging. The apprentice relented slightly, letting them both have a little air"
As you see Marek demonstrated Force choking, attacking with lightsaber and throwing objects simultaniously, while he was Force choked himself. Even Dooku's offensive TK is childs play comparing to him.
Likes of Dooku, Kenobi and Anakin are unable to break free from Force choke. Marek in comparison could break free even from Vader's grip:
"One black glove clenched, and for a moment the apprentice's throat closed tight.
He beat back the telekinetic attack with one of his own, shoving his Master in the chest with the force of a small explosion, throwing Darth Vader backward across the room"
Therein, battles are subject to nebulous elements like circumstance, motivation, terrain, and a host of character idiosyncrasies.
I always pay attention to such circumstances.
Indeed, Luke seem to hold back most of the time and use just enough Force to overcome his opponent.
But as I pointed out above it is not just immense power that makes Marek to stand out, it is how he utilizes his power during saber combat to outwit opponents.
Most of opponents Luke faced were just trying to overpower him like Unu'thul with no attempt to outwit or outskill.
Jacen outwits Luke:
"Jacen stopped cutting at the tendrils and flung a hand toward the ceiling.
"Dad, look —
Luke was already throwing himself to the deck. A tremendous crash sounded from the illumination panel, and the chamber fell instantly dark. He rolled opposite the direction he had just been moving, but wasn't quick enough. The fixture smashed into his head and shoulders, slamming his face into the deck. He heard something crunch in his nose and was instantly choking on his own thick blood."
A Sith electrocutes Luke:
"Then he rolled his blade around, brought it down, and sent a Sith
forearm sliding across the grotto floor. Whom it had belonged to, Ben could not tell.
In the next instant his father was riding a bolt of Force lightning into the grotto wall beside him. Ben ignited his lightsaber and shoved the blade into
the crackling energy, disrupting the current and freeing his father".
Yes, as you said it's all circumstantial. However, fact stays that Marek specifically excells at outwitting his opponents and noone Luke faced apart from Jacen was as good at it.