W.Stuart:
The hardness or tensile strength of the music wire does not influence the applied tension. However, the percentage of Newton loading in context to the yield point has a significant effect on sound but the small differences noted between these two scales it is arguably imperceptible e.g. The length of the C 130.8 Hz frequency over the slightly shorter Stuart will be somewhat countered by the additional stiffness due to a slight increase in tension in the Steinway. The Inharmonicity variants are imperceptible at circa 2% at this frequency level. A few pricks of the voicing needle will change everything if indeed there is anything at all.

In harmonicity produced from wire stiffness and string length variations is only a minor part of the practical application of the stretched string.
String terminals are more influential on what is actually heard and the
 acceptance of the in harmonicity levels. Stephen Paulello and I have
 discussed this matter and agree on this point. In effect, the way 
in harmonicity garb is tossed around by techo’s is mostly rubbish and
 meaningless in practical piano making as other factors have greater
influence on the sound.⁵⁰⁸ W. Stuart (email 20 Mar 2014)

The bridge agraffe reduces the heard in harmonicity factor significantly as the string is held more accurately. (email 20 Mar 2014)

Music Wire Scaling for C2 65.406Hz – An immediate difference of sound :

Reasons for the differences in the sound of the Steinway and Stuart sounding of the note C2 65.406 Hz at v20, can be initially satisfied by simply looking into the instrument. The Stuart piano uses 2 wound strings, a bichord, wound in stainless steel nickel-plated⁵⁰⁹ , and the Steinway uses 3 wound strings, a trichord, wound in copper. The Steinway piano uses Roslau piano wire, made in Hamburg, which are wound in non-tinned copper⁵¹⁰ . Roslau piano wire is the music wire of choice of many piano manufacturers of concert grand pianos. The Stuart strings are manufactured in France, by Stephen Paulello, who implements slower drawing methods of steel string production, and new composite alloy mixtures in the steel, to achieve steel of a higher tensile strength. Paulello’s innovations have enabled Wayne Stuart to extend the frequency range of the piano compass.

Samuel Wolfenden writes about the excess weight of the copper wound string,

Naturally, such excess [of copper winding weight] whether partial or total, tends to aggravate the characteristic defect of bass piano tone, viz., the preponderance of the first over-tone, often so pronounced as to eclipse the pitch of the fundamental, particularly when the strings are very short.⁵¹¹

In response to this problem, Wayne Stuart implemented changes to the standard piano design:

i) extended the piano scale of the long bridge, lower by two notes,

ii) bichord Paulello ‘M’ steel wire, stainless steel wound of greater thickness, length, and tensile strength with increased applied tension.

⁵⁰⁸Wayne Stuart, email interview with author, 20th March, 2014.
^{509     9}Paulello, accessed 21st February 2014.
⁵¹⁰http://www.fortepiano.com/wire/RoslauPiano/roslaupiano.htm, accessed 21st February 2014.
^{511     7} Wolfenden, 209.