Shiryu is as dangerous as a top tier.
I don't know if he literally is top tier as of now, but I think at the bare minimum he is as dangerous as one. Before he ate his DF he should have already been YC1. He was on par with Magellan, who was so strong that Jinbe, Ivankov, and Crocodile were all running away from him. Magellan is arguably even above YC1(IK he had bad feats up against Luffy but Oda's power levels aren't exactly that consistent).
But with his DF, he now has the ability to 1 shot almost any YC with the exception of maybe katakuri. Could haki detect an invisible man? Possibly, but haki has to be activated for it to be used. true 1v1s are frankly quite rare in the grand scheme of things. While in a 1v1, a YC might be able to put up a good fight against Shiryu(albeit I think he could beat most YCs even in a 1v1), Shiryu can assassinate almost all of the YCs.
He could just activate his invisibility, sneak into totland, and literally murder all of the veterans and 2/3rds of the SCs and probably not even get caught. Swords are deadly, when given a free shot, Killer was able to 1 shot Zoro. With his DF, Shiryu can easily get a free shot on someone who either isn't using CoO or doesn't have good CoO.
and even if CoO does counter invisibility. His powers make it to where anyone who fights him needs to use CoO constantly otherwise they're getting cut. Many characters don't have the CoO stamina or mastery to use it that much and will easily fall to shiryu 's blade.