It's very easy to calculate: a meter of 25x25x1.6 weighs:
25 mm ^2 - (25 mm - 2 * 1.6 mm)^2 * 1000 mm * 7.8 g/cm^3 / 1000 = 1168 g
For 2 mm thickness this is:
(25^2 - (25 - 2 * 2)^2) * 1000 * 7.8 / 1000 = 1435.2 g
Which is a 22% increase in weight. If we use 42 m of 25x25 and 14 m of 20x20, the increase in chassis' weight will be around 14 kg. Whereas the increase in bend stiffness of these tubes will be around 15% or so.
A much better increase in chassis torsional rigidity can be achieved by the "Aussie mods": triangulating the front frame, bridging the chassis under the gearbox where the bottom rail is cut out, triangulating corners of the engine bay.
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