when you make changes to the car (i.e: more hp, more cyn etc) you have to pit so the changes have affect. Since you are a Demo racer you much better using LFStweak, since it gives more options, and it only works on demo cars...
BTW by pit i mean shift+s, not a pit stop, just in case that wasnt clear
@ Marquise: go to single player and change between the cars, XFG/XRG/XRT, that should do it