Thanks for the reply.Two reasons why I don't want to use extensions:
The main reason is that I barely have enough room hide away the current cables nicely in my case, and extensions will only add more cables that I have to hide somewhere.
Secondly, my initial idea was to only sleeve the cables to a point where they're no longer visible, which saves a LOT of sleeving (and I'll only do the MoBo, CPU and GPU connectors, and most likely the fans (only the visible ones). In this way it'll be cheaper as opposed to buying extensions (Under €20 as opposed to over €30!)
What do you think of this method? (I'm not planning on changing my case any time in the near future, so I don't have to worry about it not working out for a new case. My current case is the Cooler Master N300 BTW)
Edit: Linus also did it this way back in the day (sleeving two cables in one sleeve) - https://m.youtube.com/watch?v=Fz7J_0fS2KU