No straight answer from me, sorry, but I remember having the same thoughts about my PSR, the weight can be a problem. I think that the main weight comes not from the speakers themselves, but from an amplifier and the speakers' casing which is also a part of keyboard's body. Also I would expect that altering the whole scheme too much may cause some mulfuction; although some people turn their arrangers into modules.
There is an option to buy SX600, it is 7,5 lbs lighter than SX900, similar to an older S670. I had to carry S670 in its box when I was moving and I would say that this difference is relevant.
But I guess you would want something much more lighter; sadly most of lightweight keyboards are either entry-level arrangers or arrangerless.