First, let's get the details out. It looks like its hinge line is directly above the elevator hinge line. And its hinge line is 30% of its chord. Then you hooked its trailing edge to the elevator so they move together. Have a got it correct so far?
First, kudos to you for conceiving, building, and then flying a change to the airplane that your seem to LOVE. That is an accomplishment unto itself.
I am curious as to how much analysis and evaluation of failure modes went into this... I will elaborate below.
Pros and cons:
Pros:
It is an increase in elevator area, and will increase pitch authority;
With the hinge line behind 25% c of the device, it will have negative hinge moments, and so decrease elevator control forces - well maybe that is a pro, maybe it is a con;
It will shift your neutral point aft a tiny bit.
Cons:
It is a pretty small added area with lots of edges, so is fairly high drag per unit increase in tail authority;
It has biplane effects - since the high pressure zone of the base tail is also the low pressure zone of the new device, it is less effective (on a per unit area basis) than the base tail;
The struts and bolts and everything hanging on it add more drag than the gadget itself does;
It had better be way secure in flight - if any of it comes loose, but it stays on the airplane, the bird will likely be unflyable;
If the control rod comes loose in flight, it may or may not flutter, but it will definitely be tough to control;
That weight increase all the way back at the tail will likely shift your empty CG aft as much or more than the shift in your neutral point.
In short, we know it works, but you have added substantial weight and added to the danger of the craft.
A slightly bigger set of tail feathers (as you were thinking) will probably be as effective, give that improvement with lower drag, and without adding new failure modes. Now that you have demonstrated to yourself that a modest increase in tail authority converts the airplane for the better, why not just buy or build that Super Cub horizontal tail and put it on. I am sure it will have less drag, is likely to add less weight aft, and is less likely to cause us to read about you in an accident report. If you fly the Super Cub tail feathers, you can always tailor the control feel by modifying the balance horns.
Billski