The flexural capacity of composite cold-formed steel (CFS) beams is often limited by premature instability of the thin-walled section, so that the compressive capacity of the slab is not fully used. This paper examines which measures remove that limit and when a higher concrete grade becomes worthwhile. A nonlinear finite element model, validated against three full-scale tests, was used in a parametric study of thirty beams. The variables were the slab cross-section, the CFS profile, the concrete grade, the type and spacing of plate shear connectors, and stiffening by web stiffeners and a lower-flange plate. Four failure modes were identified, and the toughness followed their hierarchy rather than the ranking of peak loads. At equal self-weight a hollow slab raised the peak load by 37%, and inverting the lips of the channel raised it by 78% while making the connector spacing almost irrelevant. Web stiffeners raised the toughness by up to 124%, against 27% for a lower-flange plate that gave the higher strength. Raising the grade from C30 to C60 was worth 6.6% in peak load while the section buckled locally but 12.9% once stiffened: high-performance concrete pays only after the instability of the steel section is suppressed.