The stock LC 18's weigh about 27.5lbs each according to this thread. Stock Dunlop tires are 41lbs so you're looking at nearly 70lbs for the factory 18" setup. Using the same links 16's are around 62lbs per corner stock, problem is that most A/T's for 16's are E load so there goes your weight savings. Factory offset isn't ideal for most people so you could add spacer weight to those numbers as well.
Thanks for the info. Updated my post to ~62LBS My current SCS F5 setup with Cooper MTP 285/75/R16 weighs in at ~62.5 LBS
Did the transmission change with the introduction of 18's?