I wouldn't worry about this issue too much.

First, the GCR 18 states the rollcage tubing sizes for IT are "to be calculated based on the published vehicle weight minus 180 lbs" not the actual weight (emphasis mine).

Second, there is a precedent for roll cage tubing size allowances: Spec Miata. The SM rules state that "Regardless of car weight, all Spec Miata autos may be constructed to the requirements for a 2095-pound car." Given this precedence I'm confident it can be applied to other areas.

Greg