Hmm no i wouldnt say that because the lengthy warranty comes with the 18W model as well as the 10W so for them to charge on average £10.00 more per pump over the 10W model they must do something other than what's suggested in the mod guide.
No one would never know even if you did unless you used some speical fluid and it wasn't on the original top, however it should not void the warranty and if your worried just buy one with the top that you like..