i understand in a way what you are saying but have to ask WHY? the design of the chassis has had structural tests etc & has no need for the extra thickness steel that you suggest
i,m not knocking your thoughts just not sure why change a working formula.
as for the welding required... the way i see it its part of the ethos of the concept "build your own sports car".
i couldn,t weld until i started a course at local technical college in january this year, my welding might not ( i know its not ) be to qualified cert standard but feel confident enough to build my chassis & drive it once completed, & if you build your own chassis you will "know every inch " of it which when its on the road is something to be very proud of.
each & everyone that is building a Roadster has put there own little "touches" into it whether to everyones liking or not, its your car that you built.
cheers
andy