there isn't really a whole lot to change on yours before you start playing with the laws of diminishing returns! to lose ~600 grams you'll need deep pockets.
you can shave 100g off the saddle with a carbon option, 50-100 grams with different pedals (i'll take the R540's off your hands if you do.
i presume you're already using an XG1190 cassette?
seatposts and stems can be all over the shop too. my carbon seatpost is heavier than an alloy one i have.
that said, i dont think you will notice those changes as much as something like lighter wheels, and at ~1355 grams with GP4000's they're already a pretty light package!
i'm hoping we start to see some lighter disc brake setups coming out. mine are boat anchors and i want to get mine at least UCI illegal weight wise.
Alas for my pockets. If only my bike was as light as my wallet... To be honest I don't ever expect this
bike to be sub 6kg. But I'd like one one day.
What is it with my R540s? Jason already asked for them if I ever swap them out. Fight!
Am already using the XG1190 cassette, yes. Interesting about your carbon seatpost. I know my bars are pretty light, not sure about the seatpost. Carbon saddles scare me - don't look too comfy!
I changed to Conti Force/Attack tyres to save a bit over GP4000s. I also use Conti supersonic tubes.
I have a set of Shimano Ultegra carbon pedals from before my Vectors. That change cost me nearly 100g. Wiggle
sells the Ultegra carbons for a decent price.
Interesting about the Force/Attack being a bit lighter. I guess that is an easy option next time I need to replace tyres. I might do the pedals one day, not any time soon though.
To be honest, tyres and pedals are probably about it for changes to that bike. Very happy with it now. I'd be better spending money on a cheap set of wheels and/or a separate bike for commuting. After 18,000km there is very noticeable brake track wear on my R-Sys rims...