[TalonMotorFabrication]

Slight off topic tangent warning......
Instead of looking at how to save weight on the chassis by changing tube sizes and wall thickness I would be looking at all the other parts bolted to it. A chassis weighs between 55-80kg depending on 1.5-2mm wall tube but the difference between a Sierra front upright and a Mx5 upright is about 5-7kg, then there is the calipers can they be changed for say some thing such as Hispec full ali items, solid disks instead of vented saves another 2-3kg and so on.
People get so wrapped up in the major component that they forget about the sodding great lumps of over engineered cast iron and steel that they are bolting to the chassis. If your all anal about going light find an all ali engine with ali bell housing,calipers,panels,fuel tank and end up potentially spending thousands.....then take a look in the mirror and ask your self can I lose 20kg myself? My answer to power to weight ratio.......spend another £500-1200 fit a turbo or a supercharger and stop worrying about how much it weighs.

The Roadster chassis is quite strong along it's length so what you have done with the side panels has not added much to it, how ever it's not so good across it width. There is no direct diagonal across the chassis at it's mid point but you could join the sides together by means of the transmission tunnel, if you get creative with 4-5 tubes in this area instead of the whole chassis you can make the middle of the car stronger. The only real world limiting factor to how strong you make the chassis is the human body which is a engineers worst nightmare, it stops them from adding much needed diagonals to every thing.