The flex of the collet is determined by the design, all hardness levels will flex the same.
Higher strength will give higher hardness and better fatigue properties.
Lower strength will be softer and have better toughness.
You need to choose a balance.
As quenched 6150 should be 59-65 RC.
So tempering to 42-46 RC is a medium strength condition, it sounds reasonable.
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P.E. Metallurgy, Plymouth Tube