RX vs RS where did the extra 40Lbs come from?

So I'm seriously considering getting either an RX or an RS for racing but I'm not sure if I would be better off getting the RX at half the cost and tearing it down and then building it up for racing or just getting the RS. Why is the RX so much heavier? Where does most of that weight come from? If it's just items that are getting pulled off for the track anyway then it seems like a no brainer.

I know nothing about these bikes as I've always been a gsxr guy.

Extra weight comes from aluminum wheels vs magnesium, aluminum sub frame vs magnesium, aluminum fairing mounting bracket, and other parts that are carbon fiber on the rs, body work on rs is carbon fiber also, adding it all makes the difference. If you are going to do some serious racing the rs is the way to go, the engine design on the rs will have the capability to make more peak hp than the rx because of the head design, you also have top ohlins suspension parts, upgraded dash, brake and clutch mastercylinders, etc... . Club racing I would go the rx route, body work, suspension, and brake and engine upgrades will run around 6500-8500 depending how far you go. Ohlins front and rear 3000-4000, suter slipper clutch 1200, race exhaust 1200, bodywork painted 1800-2000, ECM 300, brake mastercylinder and pads 500, tires 350-400. Rx comes out a bit cheaper, and will be about 20lbs lighter with modifications, guess you have to decide weather the other 20lbs is worth the extra 10,000-12,000, dollars.