actually, it's a hollow 1 1/4" sway bar and supposedly feels like a solid 1 1/8". I have one on my '70 convertible with Unique Performance coilovers. I don't have any understeer problems, it's actually pretty neutral and nice and stiff up front. The guy I did the order with has a '69 fastback and uses the same front sway bar.
It's hard to find a consensus on swaybars, just what you can do to correct certain behaviors. The front sway certainly flattened out my ride and gave me less body roll up front, (it now feels like the rear is trying to twist the body out of plane, even with subframes

) but what exact behavior are you trying to correct?
I have a pet theory that convertibles need bigger sway bars to keep front and rear from body rolling relative to each other, and that they respond better to changes in sway bar because differences in body roll front and rear unsettle the convertible frame a little more . . . but you have a coupe, so not as big of an issue there . . .