Hi Nathanael,
I agree, a car loading HSC would be great. Guess, the most suitable fast craft for several existing areas would be one of the wavepiercer type, as your mentioned Seacat or Lynx. The HSS 1200, serving Hook van Holland - Harwich, was withdrawn from service a few month ago and the somewhat smaller HSS 900 "Stena Carisma" connects Sweden with Denmark. But wavepiercer type fast crafts are not as purpose built as the Stena ones.
Cheers