The only time anything will come into play with the finance company is if you voluntary terminate (VT) the car or have it re-posessed. They could charge you to turn it back to standard if they thought that the modifications de-valued the vehicle or if it was dis-tastefully done. Depends on each individual company though. Wouldn't worry too much, I know a few people that have sent the car back slightly modified (typically bodywork or alloys) and not had any problems. It's up to you to decide though.