If you keep the same planform to skeg and rudder, improvement is likely to be too little to be noticeable, even if you were racing other Tiki 21s. If you install kickup rudders with a good profile
and deeper draft, you can get, as you say, more lift and less resistance. However, I expect Wharrams to be designed to get only a modest proportion of leeway resistance from its rudders, both because they are on the small side, and because skeg-hung rudders have no balance, and loading them up increases loads on the tiller. So if you keep the rig as it is, you improve the lift to drag ratio of a part of your lateral plan that contributes only a little to lateral resistance. Your overall leeway would not change, because more lift from the rudders would simply make you fall off. You would have the same leeway, but with a little less drag from the rudders. Though if you make the rudders so large and deep that you reduce their angle of attack to, for example, 1 degree, you will have a low lift to drag ratio again.
If you want to get real benefit, you would have to copy Hobies, which load up their rudders by moving the rigs aft. They do that by mast rake, because that boat also wasn't originally designed for this. The Tiki having a boomless main, you would need one with a higher clew.
You might want to reinforce the stern a little, and if you want to keep the shape as designed, take into account that the steering axis of your kick up rudders will be raked forward. Check the arc of your rudder blade tips when hitting the ground. If the tips arc down, they must lift the stern while kicking up. That would introduce vertical loads that your rudder fittings may not be designed for.
AYRS (Amateur Yacht Research Society) booklet #95, which you can download from
Booklets – Amateur Yacht Research Society https://www.ayrs.org/booklets/, contains a report from a Tangaroa owner who put small endplates under the hulls. That would reduce leeway and may, if you get the shape right, improve the lift to drag ratio of the hulls, when going upwind. When there is little or no lateral load, you would have more drag.
Bernd Koehler has used that idea on some of his designs.
You could keep the Wharram rudders and make them more effective by adding an endplate. As above, to get the most from this you would have to load up the rudders more, and with skeg-hung rudders, that increases the load on the tiller.
In summary, you should be faster upwind and have less leeway if you use deeper rudders with better profile, and redistribute the lateral loads away from the hulls and onto those more efficient rudders. You may be able to improve the lift to drag ratio of the hulls a little as well. These would be the smallest design changes I can think of to improve upwind speed. You may lose some downwind speed.
