Word of God is that Omi's aim is so good he can hit an apple a kilometer away. Similarly, Youji's wire can pierce a 20mm thick iron plate, according to Tsujii's files.
Takehito Koyasu originally wanted to call the series "The Cat People." The producers told him he needed a cooler, stronger-sounding name, which made him think he needed something in German. He chose "Weiss" because it also sounds like "vice," indicating the fact that Weiss aren't actually the good guys.
Youji and Ken were originally supposed to have 'second' names, like Aya, Omi, Sena, and Chloe do. Ken's was 'Hyde' and Youji's was (naturally) 'Asuka.' Koyasu's admitted that he likes giving male characters feminine names.
One of the series' original inspirations was a period drama about assassins in Edo called Hissatsu Shigotonin. Ken and Youji's weapons were both inspired by this show, and Tomokazu Seki is also a fan.