[Swampdude]

From our 2004 experience I personally won't go under 1/8" exterior structure (actually 3/16 minimum for me, but I don't use box tube). It's true you could grid a well placed backing that would take the hits. But you'd need at least the amount of material you're saving by dropping the wall thickness. This is just an opinion. I'm not doing any analysis to prove it. But I've watched .090 cave in like a beer can. Then if you open up the walls like in the photo at an unsupported mid span, you're definitely creating a hot spot.
If you have CNC or water jet support, I'd recommend you keep the thicker wall and take truss shapes out of the tube for lightening.
Another thing that bothers me about what you're doing is using this tube for your drive train features that require good alignment. If it starts deflecting the axles or gear teeth (if you integrate the transmission like you did last season) would become misaligned. The last thing you want to do in this game is compromise your drive train.