Competitive weight is always going to be as light as the class allows. However, competitive weight may not be what your truck, driving style or, track may want.
If I get the right idea your trying to lighten a pre existing kit or build your own. If this is what your after and still keep it ROAR rules, try taking the wight of all the gear (Batteries, ESC, motor, RX, Servo, tires, transponder and, even body)*as well as all the drivetrain parts if it is a DIY truck* and get a wight on those. Then subtract that from ROAR rules. This will give you good idea how much of a diet you can put the truck on. Watch where you take out wight. If you cut the wrong places, the chassis will flex like crazy. That will make the truck act real funny on the track. And make sure not to cut so deep it brakes in half after the first landing.
I've been there.
Carbon or graphite chassis is always been a good upgrade in my past experience. If none are available there are places that will cut ones based on the info you send them. That is all in how much you want to spend. Me, I'm allergic to spending money.
Doing small things like cutting out the center of diff outdrives or slipper pads can help bring not only static weight down but also rotating mass. Good for quicker lunches. In lose conditions this can make the truck harder to control. Carbon rods can for idler gear shifts, top shafts and center shafts if your good with making parts and your not running to much power to them.
Maybe if we had an idea what truck you where specking of, we could help better.
Hope this helps some.