Your readings are going to be higher with the now shortened spokes due to the wind vane circumference is smaller, the distance traveled per revolution is less.
The wheel-speedometer example is not a good analog to an annemometer for a couple of reasons....
It is true that putting a smaller than specified circumference tire on a vehicle will increase the inaccuracy of the speed measurement, but putting a wider tread (analogous to larger windcups on shorter/longer arms) on the diminished-circumference tire will not compensate. Direct contact with the road turns the tire/wheel/speedometer and increasing friction or drag (eg, by leaving the parking brake engaged) will not change the relationship between the wheel and the road unless it is completely locked up and skidding. It is always a 1:1 relationship to contact. Only the formula used to translate a rotation count to speed can compensate for the radius (vis circumference) change.
An anemometer on the other hand has various factors contributing to torque and drag, and therefore the acceleration and spin of the unit relative to actual wind-speed. Those can include length of those cup-arms (eg wheel diameter), size of the cups, shape of the cups, drag of bearings, etc. Changing any of those could have a measurable effect that could only be accurately measured with a fixed wind source. I'm sure a lot of thought goes into the design of an anny such as
x rpm=x mph using a look-up table, which wxtech's anny knowledgebase thread seems to indicate.
In it's most basic form, a big sail catches more wind and moves a boat faster than a little sail, but the wind still makes it billow if the boat is anchored and not moving. The road doesn't cease to move by a rotating wheel at any rate other than they always make contact, unless it is skidding.
(sorry if confusing, I just see the wheel/speedo analogy as apples and oranges)
I say if your mod provides a more accurate reading by your observation as compared to what you consider a trusted standard, then great! I know you will watch it at various speeds to see if it tracks.