*People saying Guilliman vs Abbadon would be a close fight*
Well why not? Are the Primarchs somehow unimpeachable in their power? It makes the most sense from a narrative and thematic standpoint (not to mentions his wargear alone being crazy-powerful). Anything else seems like a weakness with following through with the narrative and progression of Abaddon's character just to keep the Primarchs special snowflakes for fans.
honestly? yeah it matters, in talon of horus, when they have their rumble with clone horus, it takes a decent sized group of the most badass of badass chaos marines to take him down, people say abaddon killed him - abaddon landed the finishing blow, but it took them all. It's naive to think otherwise. Likewise, Abaddon only beats sigismund because sigi is old and slow, and even then sigi almost wins, and all the chaos marines in the vicinity are "dealt with" to hide that fact in the aftermath.
HH rules get it right, no marine, even sigismund at the height of his ability can defeat a primarch in one on one fighting. To have that change will actually not be good for the setting, regardless of what some might desperately want.
Should Abaddon be able to beat Calgar? Yeah, I would say he should be able to defeat any loyalist, except for Guilliman, whom I think he should be able to badly bloody, but not defeat.
However, that is all without taking into account Abaddon's current power. He was without the Murder Sword against Horus (and still managed to gut him with all the Primarch's advantage in speed and resilience), and with both Horus and Sigismund he still wasn't the explicit Warmaster of Chaos, chosen and empowered of the gods. Sigismund is the pinnacle of what a Marine can be, but Abaddon has broken that rule because that is what you can do with Chaos - break natural physical rules and limitations. The prices and risks of doing so don't need to be mentioned, but Abaddon's entire character worth is around him overcoming all that.