Although wing tip plates are fun to experiment with and I have played with them in the past (I put them on a Ken Willard .09 powered pylon racer design in 1964, kick up escapements for elevator) it is a real problem determining the effects in a model world.

We spent several days in a wind tunnel once and as we were looking at gross effects our devices were big, if there was an effect we would refine them later. Large fences were put on top of the wing and we varied the lateral position out to the wingtip. It is surprising how little effect was seen.

On a typical model to get the same lift just reduce the weight a few percent, increase the angle of attack or increase the control throw a little. A small decrease in taper ratio will give the same effect. Since most aero forces and moments are a function of V^2 it is easy to get the same effect as tip plates on the high velocity side, just a little more throttle. Putting tip plates on a rectangular wing is helping something that is good to start with.

On the low side is becomes a problem of placebo effect. If you think they are helping then they will appear to help. We are all human and that is just the way we work. Other than having them for the fun of it to really be sure they are working do this test.

Determine a setting of a low engine throttle that can be duplicated each flight by counting the engine stick clicks or by putting some sort of repeatable stop to assure the same engine speed. Do the same thing for the elevator control.

Set the engine and throttle for low speed level flight. Do this with and without wingtip plates. Make every effort to keep the stick settings the same. Do a long level pass the length of the runway.

Since the tip plates will increase the lift coefficient of the wing by stopping the loss at the tip if they are working as expected the airplane should climb with them and not climb without them. If you can't tell the difference then the differences are buried in the experimental error and you wouldn't be able to determine the difference by just flying the airplane around.