It's a pretty big leap to go from 2D scrolling shooters to pseudo-3D chase-view shooters. The point about Sewer Shark is that like Hyperion, it's outside the normal scope of the genre. Does anybody ever bring up Space Harrier when they talk about shmups? I don't think they would even let you post a high-score thread for it at shmups.com.
Silpheed is admittedly also kind of a borderliner, but the gameplay itself takes place almost entirely on a 2D plane. I'd say it qualifies as a gimmicky vertical shmup.