If we examine a 25x25 RHS beam, 2.0 vs 1.5 wall thickness gives a 23% increase in stiffness in return for a 30% increase in weight. In other words, it adds only about as much stiffness as it adds weight.
Work it out for yourself. A beam deflection calculator is available here: http://www.calculatoredge.com/civil%...lator/beam.htm
, and the weight is proportional to the cross section area, which is d^2 - (d - 2*t)^2, where d is the tube width and t is the wall thickness, if we ignore the rounded corners.
Speaking of the added weight, theoretically it sums up at 15.9 kg for 36 m of 25x25 and 14 m of 20x20.