Here is what I have done. Mine is down to just over 3400g. Body, AKA Tires, and Empty Tank... As measured at JBRL...

Titanium Screws : 4x12s on steering posts and radio tray/front battery box attachment (I would replace the ones that mount the bulkheads and radio tray to the bottom, but Ti strips out and the heads get messed up easily if mounted on the bottom. The solution for using Ti on the bottom screws is to have the chassis holes countersunk 1/2 mm deeper so the screws sit up in, rather the flush with the bottom of the chassis)

Aluminum Outdrives: They haven't wore that badly yet http://www.amainhobbies.com/product_...eshaft-Axles-4

LiFe battery/ KO 407s receiver

All Servo mounting screws, and any non weight supporting screws have been replaced with aluminum ones. The 2 screws that join the bulkheads below the shock tower. The ones that hold the sway bar attachment on the bulkhead. All the Shocktower mounting ones to the bulkhead. (You get the idea) Also all the M3 nuts and M4 that screw on the shock tops are Alum. http://www.fastener-express.com/AluminumScrews.html

Ti - Shock Pins on A-Arms (Lunsford) http://www.amainhobbies.com/product_...k-Mount-Pins-4

Ti - Upper Hinge Pins on front arms, can't use the other ones as the outers bend easy, easier then the steel ones, and the newer inner ones are bigger and bead easily, has some tops laying around (Lunsford) http://www.amainhobbies.com/product_...-Hinge-Pin-Kit

Alum Steering Posts: King Headz, but these sheared on the first outing so back with the steel http://www.amainhobbies.com/product_...r-Posts-1-pair

Plastic Chassis Braces

Turnbuckles: Ti all around

I think there might be a bit more, but thats all I can remember.

Future Mods:

BCE Chassis
Ti screws for bottom side
Graphite (B) Towers when I can get the PSM ones.

I think with those mods I'd start to be underweight, so would stop there.