i can certainly see the copper tape causing resonance issues since it's a thin slab of sticky back copper being stuck to the guitar. it's like a scaled down version of using dynomat or hush mat in a car when doing a high end stereo to stop things from resonating and rattling. so i'm not a big fan of copper tape in guitars. however i've never noticed any difference in resonance when using shielding paint and i have it in pretty much every guitar i own and have owned in the past (i like them extra quiet). it's actually a very thin fairly watery paint and not some thick heavy paste like stuff or anything like that. thus why i've never noticed any resonance change, just quieter guitars. the thick ass poly paint on modern guitars will cause more of a resonance change than a thin layer of shielding paint will that's for sure.
-Mike