Factory metal wings and aluminum gear TOGETHER increases max gross by 150. Either component alone does nothing, legally. Prior to the aluminum gear in 2004, there was a period where the max gross increase could be obtained with metal wing, extended spring steel gear, and upgraded bolts.
ACA webpage lists nominal empty weight for current 8KCAB as 1304 lb. My 1978 model has nominal empty weight of 1330 lb. The max gross increase plus weight decrease would result in a useful load increase of 175lb, enough to legally take off with 2 grown men and full fuel.
From what I have read, the max gross is determined at least partially by a drop test. The aircraft is loaded and dropped from a height of 4 feet to see how much weight would cause a prop strike. Since ACA only supports metal wing aircraft, they have never done the drop test for wood spar aircraft. If they did, they would likely get the same improvement. The fact that aerobatic max gross is unchanged suggests that the wing is not the limiting factor.
I believe, but do not know, that the aluminum gear are longer, stronger, stiffer, and lighter. The aluminum (or extended steel) gear also permits a 2" longer prop, such as the Extreme Decathlon uses.