Very unlikely.

The only potential danger I can think of in adding side area like that is if too much area is added forward of the CG. In that case, it's possible to have divergent yaw stability (ask me how I know). Your extra side area is distributed fore and aft of the CG so I don't see any controllability issues with your mod.

Those plates have at least two unrelated functions: Inhibition of spanwise flow and additional lateral area for maneuvering. Judging by the large size of your plates, you were going for both, which is fine.

Based on an article we have all read, I am considering adding some much smaller temporary tip plates to my Giles, to see how they do. The plane already knife-edges extremely well, but the claims that tip plates can reduce or eliminate wing-rocking in harriers and clean up snaps have intrigued me. I can do decent harriers with this plane but they are not easy, and the snaps could do with a bit of cleaning up.