Yeah, I'm familiar with their early (brief) role as Marine Hunters. I'd like to see that come back.
Space Marines are a pretty risky resource desu. Even good, straight laced loyal-types sided with the Tyrant of Badab. So having the Ecclesiarchy's elite warriors also specialize in terminating rogue Space Marines makes decent sense. They serve as the Ordo Hereticus' Chamber Militant, making that a perfect fit too.
But, eh. GW would probably balk at the idea of showing a group within the Imperium that could be a viable threat to the Space Marines. I'm not asking for them to be superior, or even equal to the Astartes, I would just like them to be a potent threat that the Space Marines have to be wary about. This way you can develop all sorts of fun politics and tension. The Black Templars are probably totally bro-tier with the Sisters, while the Dark Angels hate them and often avoid deploying on fronts with a large Sororitas contingent. >>46386676
Personally, I advise against getting in to the wargame unless you have a group of buddies who do it. Try the rpgs instead, and buy minis that will be fun to use for combats in that over full blown armies. The game is simply too expensive and too poorly balanced to be something I can genuinely tell people to invest in.