This may or may not help but I have personally pulled apart my SX900 to replace the grease on all the black keys.

The speakers are not actually that big so I dont think it will save much weight at all by removing them. Where most of the weight comes from I think is the wooden casing around the speakers. you could probably remove all that but It would most likely involve separating / cutting through the casing of the keyboard... If you were handy at crafting - you could easily replace or cover any gaps using aluminium / painting black to match etc...

It may sound scary to some, but not that hard to accomplish if you have tools and the know how...

good luck!