devtrackers.gg
These nice folk have done a great job of explaining many of the finer details of Siegfried. I will add a few specifics about the matchup between Atlantico and Sieg:
Siegfried's bow/stern armor is 27mm. This means your guns must be 406mm or bigger to overmatch
Siegfried's Deck armor is 30mm. This means your guns must be 431mm or higher to overmatch
Siegfried's upper hull is 90mm. Nothing overmatches that, so the Siegfried will bounce AP shells of any caliber if she's angled enough.
Siegfried is classified as a "Large Cruiser" in WoWs-Speak and will burn like a battleship for a full 60 seconds.
Atlantico has 381mm guns. This will overmatch 26mm, but not 27mm. Atlantico's AP shells will bounce off the bow and deck of Siegfried if she is angled.
Atlantico's 234mm secondaries WILL penetrate the dock, bow, stern, and superstructure of a Siegfried at any engagement distance.
Atlantic's armor is not overmatched by Siegfried's 380mm guns.
Since Siegfried is a Tier 9, but a cruiser, you may think you are on pretty even footing with an Atlantico. That's probably fair to say. The Siegfried will do best staying at mid-long ranges and utilizing AP/HE as desired and making use of exceptional accuracy to score hits on Atlantico. The BB fire duration of Siegfried is not a worry against an opponent that can only fire AP from the main battery.
Atlantico's strength comes in her incredible secondary battery array. Her guns are fantastic and that's what she needs to use to win a duel with a Siegfreid. You will need to get within 11km (ish) to consistently have your secondaries firing, but you need to make sure you do not close to within 6km, or the Siegfried can make a a torpedo run on you and that's all she wrote.
Hope that helps!