3mm is not over kill... it is the stiffest to go without weighing to much... 2.5mm flexes more but weighs less... 3mm is ultra rigid.. the 3.3 mm u speak of was probably rated as 3mm.. they are not perfectly flat.... ive seen 3mm from .117-.133
i had a pan car chassis that was 3.5mm... it rated .133-.142... it was way to heavy.. ultra stiff but to heavy...
the 3mm also allows u to mill the chassis out more for less weight. if u did the same with 2.5mm it would flex a ton... for most applications u want the stiffest chassis..