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@gbj_tester
You can't get around the fairly simple mathematics. Increasing the stiffness of a steel tube is quite easy - increase the OD, increase the wall thickness, maximise the moments in the required directions (at the cost of those in non-preferred directions). The particular steel in use is all but irrelevant until you get to such thin sections that UTS becomes problematic, e.g. you need 753 to make usable tubes 0.3mm thick, but at 1mm thick mild steel is strong enough. You can get clever with FEA to ensure that you get the butts in the right proportions, and you can make surprising large changes to lateral stiffness of the rear triangle with quite subtle adjustments to the geometry and the bridges/gussets/fillets, but really it's 90% down to just using more metal if you want more stiffness. All that being said, people's subjective impression of the stiffness of a particular frame is very nearly the worst possible guide to how stiff it actually is, only marginally second to looking at pictures on the internet.