Been a while since I was involved in karting, but I thought mild steel went out in the 70s because you could use up two to three chassis in a season. The thing that made Chrome molly better was it's memory, but even at that on factory teams we changed them out a couple times in a summer. Even their memory isn't perfect. I'm not for Chrome molly for a lot of things, but in a cart, it would be a waste of money not to have it. The guys are right a piece of 1 1/8 .083 in mild or molly weighs the same. There is a hardness tester kind of a spring loaded center punch that records the hardness. Not sure of the name or the price. The only other thought, would be to make templates off several of the legal karts. It would be very hard to reproduce the bends on a different mandrel. Hopefully the maker held to good tolerances. Of course on mild steel a crash could change some measurements but it wouldn't change them symmetrical. The best way out and cheaper for all in the long run would be to supply a C/M kart.
I doubt this is the reason they are fast. We had our Karts for sale at the race at all times after competition. It went less engines and seat and seat hardware. We was always competitive with them the next week. That should tell you where to look for advantages. That don't mean they are illegal.