You would have to make minor adjustments, I would estimate the effort to less than 5 minutes per track. Basically it only means adjusting corner angles by +/- 1° and maybe scaling the track length to your desired value. Not a big thing, but if you don't do it, the car will visually go off track on longer circuits. The Hockenheimring less than 10 minutes to make from scratch without time-lapse (it was set to x8 in the video).
at Packbat: No you cannot split track segments yet. I don't think first making a rough track and than refining it working properly, butI see the situations where you would want that feature. It shouldn't be too hard to do, I'll give it a shot.