Even if you get 1/3 Kurt's gain it's worth the effort and cost
First let me say that a 9 kt increase in speed is more like what I would expect from no fairings to wheel fairings and it is more that twice the improvement that I achieved with my best speed mod. However, this is an example of actual experience that validates improvement from the change.
When we built our RV-6A over an 8 year period, Van's was in transition to new cowl, new vertical stabilizer/rudder and new landing gear fairings. We got the new cowl, old vertical stabilizer/rudder, new fiberglass main strut fairings from Tracy Saylor, new fiberglass nose strut fairing from Van's (I think this was a special order), new main wheel fairings and old nose wheel fairing.
In trying to improve the speed I developed subfairings to go on all three wheel fairings extending the bottom down to 3/4" above the ground. The addition to the new pressure recovery main wheel fairings made a fraction of a knot increase in speed but the addition to the old flat sided nose wheel fairing resulted in a dramatic 3kt gain in aircraft speed. I think this experience also validates the relative inefficiency of the older flat sided design, though in a different way.
Bob Axsom